Nope, never tried it, and with reason. First off, I run two different tire packages, all in 275/35-18. Dry, they're Hoosier R6, and wet/street/"screwing around", I run Dunlop Direzza Star Spec.
To my mind, it's all about weight transfer, load, and torque. I should have mentioned (kinda forgot, it was a while ago!) that I also ran HP+/HP+, and it was pretty miserable. Think of it this way: When you get on the whoa-pedal, quickly and smoothly like you're supposed to, a few things happen, all at the same time. First, the rotors (front and rear) start to decelerate, and due to good-old-Newton, we know that the chassis wants to continue, but can't, seeing as it's bolted to the rotors that are decelerating (through the tire contact patch, of course). This alters the loading on the front and rear pairs of tires, increasing the load (and thus available grip--to a point) in front, and decreasing the load (and thus available grip) in the rear. If you ran completely identical brake packages (rotor diameter, MC piston area, caliper piston area) front and rear, you would pretty obviously get to the point where the rear tires would simply lock up, while the fronts were still working beautifully, decelerating the car. If you have ABS, this would be the threshold point where the system kicked in, and would start pumping the brakes for you. Net result: The ABS is in too soon, thus limiting your total braking capability. The OE brake package alters the relationship of MC piston surface area, caliper surface area, and rotor diameter, reducing the amount of braking delivered by the rear brakes to something approximating the same percentage of bias split as the tires are seeing load. I'm pretty sure that they picked some nominal value, such as 70% of maximum braking pressure for the bias calculation, assuming that that would be an "aggressive street driving" profile. I think they rely on the ABS to work out the fine details under a panic-stop condition.
Now, we have a small problem transitioning this over to track-style driving. Assuming a "square" pad compound setup is ideal for the aforementioned 70% braking effort, which would also correspond to roughly 70% of the max load transfer, things change just a bit when you get one of us maniacs behind the wheel. I'm pretty sure they didn't develop the brake bias ratio with a "Drag race! GO! GO! GO! Crap, corner coming, stand the car on it's nose, oops, too late on the brake, gotta turn in under braking, there goes the rear end, quick, catch it with the gas, okay, now power! power! power!" mentality in mind. Since we're actually trying to GET to that 100% maximum possible braking, with corresponding load transfer, that means the rear tires will be even lighter than the system is "programmed" for, and we start limiting based on the rears locking up again. Thus, we track rats run differing compounds (1 "notch" down in the rear) to help "reprogram" the baseline bias a little further forward.
Now, in my case, I found a 1-compound split (XP8/10/12 or HT10/DTC60/DTC70) was about perfect with the stock brake hardware and stock ABS software. I regularly ran right at 1.0G under braking, but never saw over 1.08, as the car was deep into ABS at that point. When I changed up to the Brembo calipers and 14" rotors up front, that altered the equation drastically. With the extra inch of diameter (plus the considerably larger swept area under the pad), I again had bias issues, but this time with not enough rear brake, and I could again trigger ABS, but this time at around 0.92, when the fronts started to lock up with the greater brake torque. I went with the DTC-60 compound on all four corners, and suddenly, I had my braking mojo back. I can't honestly say that I could feel the difference between split and square compound setups in terms of actual front/rear bias, but I did notice a difference, in that the split setup just felt "off," like the car wasn't decelerating like it should. The square setup just feels great. Now, if Hawk would produce our rear brake plates with DTC-70 compound, I would go right to 70/70, and really enjoy the additional initial bite.
Does any of that make sense? I know it's all anecdotal, excepting the traqmate decel data, but I would be curious what you're feeling that makes you think that your rears aren't working hard enough. At least with my braking style (late/hard), the compound split is good with the stock brake hardware, but needs to go one compound more aggressive in the rear to compensate for the extra torque the 14" setup generates.
