Not sure what you mean by "flop" of stock seat.
If you grab the stock seat and push/pull on it, you'll see quite a bit of give to it...
Alum option - I had been warned off an alum seat for safety reasons. The theory was if there is an impact that reaches the seat, an alum seat would bend and possibly hurt or pin the driver.
Seriously, take a moment and think about that. For the seat to bend enough to pinch/pin you, that implies that there is significant amounts of movement room. Think front impact: The front of the car stuffs itself into a tire wall at, say, 70mph. The car comes to a (sudden) stop, but YOU are still in motion (inertia).
With the stock setup, the pyro fires, tensioning and locking the belt, which due to it's length, begins to stretch. Honestly, it does! Your whole body is still moving forward, but decelerating rapidly. At about the same time, the front airbag deploys, and your torso and head have a (relatively) soft pillow to smack into, completing the deceleration. Leaving out crumple zones and crush space in the chassis, the net force of the impact has been dissipated by the safety system over both time and distance.
Now, let's look at the same scenario, but this time with aluminum race seats, 5-point harness, and a head/neck restraint, like a DefNder or HANS. The car still hits the tire wall at 70mph, and you start flying forward. The first thing that happens is that the anti-sub strap(s) pull down on the lap belts, forcing them down onto your pelvis, and away from the all the nice, soft, squishy organs, like liver, spleen, etc. Now, at this point, your pelvis is pretty well locked in place by the belts. What about your torso and head? Well, if your belts were tight, like they were supposed to be, you will get a minimal amount of belt stretch (much shorter overall length than OE means less net stretch), and then become completely taught. This does several things. First, it locks the horsecollar of the head/neck restraint down over your collarbones and onto your ribcage, spreading the impact energy over a wider area, but again on solid bone, not squishy bits. At this point, your torso comes to a stop, with all the energy thus far transmitted across bone. Your head, though, is still in motion, but not for long! As soon as your head (helmet encased, of course!) reaches the limits of the head/neck restraint (call it "HNR") tethers, your helmet (and your skull, and your neck) will STOP moving forward, as the force is redirected through the tethers, and down onto the horsecollar, where it meets, you guessed it: BONE.
With the "race" setup, you will experience higher peak G-load (decel) than with the OE setup, but there's a higher safety factor in that the seat and belts have positioned you to take the entire impact on bone, with no organ involvement. Second issue is that of a glancing blow, which in an OE setup will trigger the SRS, and deploy your airbags, which may not be what you want to have happen when you're struggling for control in a car that's ricocheted off a wall and is now sideways at high speed...
Let's back up for a second, though, and take a look at the seat, and what was happening with it through this crash. For it to fold enough to crease and pinch or trap you, it would have to move quite a bit, but where is the space for that motion? All the force is forward, and your back will pretty well prevent the seatback from folding forward. Let's go worst-case scenario: the seat itself completely tears away from the mount! Where's it going to go? Not forward, since you are actually restrained by the belts, and there's not enough mass to the seat to do any real damage (low mass over a VERY large surface area: back, butt, thighs, calves).
Now, let's turn the crash around, and suppose that you spun and backed into the tire wall at the same speed. With the OE setup, all you have to keep your torso in position is the locking mechanism on the recline function of the seat. Granted, even if the seat back fails, you'll dissipate the bulk of the energy transitioning from torso-vertical to torso-horizontal, and the headrest will keep your cervical alignment well enough to stay among the living.
With the race seat, there's no recline function, however, the principle still applies. The side bolsters (or the whole seat pan) would have to tear out to let the seat back "fall" into the horizontal plane, but you're still dissipating energy. Now we need to touch on a little bit of tech. FIA homologated seats (like the $1000+ Sparcos, etc.) are engineered with a certain amount of flex to the seat back, that is designed to trade distance for force. Non FIA seats, like the aluminum Ultrashield or Kirkey seats, REQUIRE a seat-back brace connected to the rollbar to prevent extreme rearward motion. The force applied to your body is still spread across bone, but even moreso than with the stock seats. The bulk of your mass will be acting directly inline with the seat-back brace (I use the I/O port setup), which transmits the force from you, through the seat (with little motion to speak of), through the brace, into the rollbar, and thus into the chassis of the car. If you're worried about the bottom of the seat coming loose and subbing under the bar, you would have to have FOUR (or more) 5/16" bolts, with fender washers to spread the load, tear through the aluminum seat in shear, which just isn't going to happen in any kind of a survivable crash.
In short, though, what you've been hearing is either an old wive's tale (wearing a seatbelt could trap me in a burning car, so I won't wear it on my way home from the pub with a six-pack onboard...), or the result of some improper installation, like no back brace, or undersized hardware (with no load spreading). The physics of it just don't work. Look at NASCRAP, where the guys crash all the damn time. What are their seats made of? Some run FIA carbon-fiber seats, but most run aluminum... I seriously doubt that any of us, running in S197s at club tracks, are going to be exposed to 180+MPH crashes into concrete. If we build the car up enough to be capable of running those speeds, most likely it will already have a full cage in it.
It was actually 2 near misses. The sway bar link ear broke off and sent me pointing towards a tire wall. I didn't realize what exactly happened and went along my merry way. The lap after that was when the tire got ripped up during a turn and if turned out to be the outside tire that blew, I don't know if I could have managed to keep the car out of the forest.
