I just don't see the performance advantage outside of maybe a slightly better ride mounting the rear spring on the shock, seems mostly to me paying good money to solve a problem that doesn't exist, which is probably why MM and Vorshlag don't sell them, sort of like torque arms for these cars.
They're just another way of achieving roughly the same thing. But minor as the end result differences may be, they might still be there. Here, mounting the springs over the shocks in the rear allows you to run with a smaller rear bar in order to achieve the same roll resistance in the rear. That means you have less side to side crosstalk through the sway bar over single-wheel bumps.
It's a difference, small as it may be. Especially with a stick axle, I expect it's not much of a difference, but it's a difference nonetheless. It's probably dominated by other factors in real-world testing, such that only the most talented might see an improvement from it.
In the end, the solution of mounting the springs over the dampers in the rear is more of a "niche" approach than maintaining the stock locations, and I expect that's the primary reason Vorshlag and MM stuck with the stock locations -- not enough of a measurable gain to make the extra expense worth it.
Keep in mind that Filip Trojanek is a mechanical engineer by trade. It's likely he settled on the solution he did because it is
in principle a more pure solution, and that will certainly appeal to the engineer in him. And since his primary target market appears to be the "no compromises" crowd, it makes some sense for him to go that direction.
Vorshlag and MM appear to be oriented a bit more towards the mainstream market, which of necessity means greater bang for the buck. They have products that overlap the market Cortex is targeting, but they sell to a more general audience as well. Because their markets are wider, they almost certainly have to account for that fact in their lineup. You don't see them offering things like SLA front suspension setups, or Penske or Ohlins dampers (for those who regard JRi dampers as cheap junk
), the way Cortex does, and I suspect it's because that market is too much of a niche market (not to mention that, for all I know, the MCS dampers may well achieve the same level of performance as the Penske or Ohlins dampers do). Conversely, you don't see Cortex offering any standard strut systems like Vorshlag's Bilstein StreetPro setup, because that's outside of Cortex's target market. Cortex appears to be going for more of a performance-without-compromise approach (their decision to locate their shop at Sonoma Raceway is consistent with that), which of necessity reduces the "bang for the buck" factor. And while they also seem to have some products that target a more mainstream market (their lowest end coilover system uses Koni dampers), that doesn't appear to be their
focus.
Back to the question at hand: the location of the rear springs. If the performance difference between the two approaches were massive, I've no doubt that Vorshlag and MM would pursue the one Cortex is using. But if the difference is sufficiently minor, then going down that road just wouldn't be worth it for them -- it adds noticeable cost for what is likely a very small gain. After all, you can achieve the same roll stiffness in the rear, while maintaining the same bump stiffness, by using more rear bar.
I like that there are multiple solutions to the problem, and that one can choose where on the spectrum of price versus performance one wishes to be. I also like that simple people like myself can have immense fun with their cars without having to go crazy with suspension mods.