I agree on it not being worth it to install the E93 bar. The H&R bar though is 42.52% stiffer and...
Here are some thoughts I posted in Richbot's autox thread about bar stiffness comparisons:
I tried to run some numbers on the stock versus various aftermarket options, but the stock bar is hollow, and without cutting one in half, I don't know the inside diameter. So I ran across a FAQ on Turner's web site on the H&R front bar where someone supposedly did the calculation for a 27mm solid H&R versus stock, and they claim that one is 1.425 times the stiffness of the stock one. I assume the moment arm is equivalent to the stock bar.
I'm looking at all of this since I ended up buying a Dinan 28mm solid front bar on sale for 40% off, and I'm wondering if this will prove to be too much front bar on street tires. Interestingly, Dinan adds this bar to their spring kit (i.e. moving from their stage 1 to stage 2) without changing the rear bar, so clearly a car with just their springs versus springs + front bar is going to be a good bit different, but they must feel it isn't too much bar. All that is an aside...
So if I assume Turner's H&R vs stock calcs are correct, then the Dinan 28mm bar (same moment arm as stock) should be: 1.4252 x (28^4 / 27^4) = 1.65. Hence it (Dinan bar) is 65% stiffer than the stock bar.
Direct link to post in Rich's autocross thread
2011.5 M3 sedan 6MT, Silverstone, no sunroof
2001 M5 LeMans Blue/Silverstone
2006 330i 6MT Red/black, ZSP, Nav, etc.
2001 Audi S4 6MT; 1996 Volvo 850R wagon