The main reason I elected to give the fiberglass leafs a try, back in the lat 1980s, was primarily for reduced weight -- remember, I was in serious SCCA Solo II/autocross competition. Reducing the weight here, there, everywhere "legal", was a main goal for continuos performance improvement. Flex-a-Form was helpful in my discussion with them to determine the puonds/inch and the arch, but they needed to know the rear corner weights of the car (left-rear, right rear), with my driving intentions (95% solo II/autocross, 5% street... back then) to make a best conclusion. So, although I've only tried the one size (5" arch with 225 pounds/inch), I'd be curious if a little stiffer would be better. I know that below 200# and less would have resulted in increasing the free arch to achiveiheve a near flat compressed arch, but I took what they suggested and eventually played with the adjustable red KONI shocks to find an acceptable performance ride in the autocrossing. Overall, I'd say that I've been satisfied with the results, even for increased street driving that I've been doing in the recent years. I do remember that they supplied rubber front bushings, and it was a hassle getting that material out, and that I had to slightly decrease the OD of the urethane bushings to make those fit into the front eye of the new leafs... the rear was no problem for urethane. Also, I know that some other SCCA national solo/autocross racers were running the fglass leafs on their Camaros, FBirds and Mustangs... all with about the same 225 pounds, a couple were a little stiffer... then, some of the C/Prepared pony cars (all-out race) were stiffer up to ~300.. little free arch and still flat.