I've been running the Flex-A-Form fiberglass mono-leafs for my SCCA Solo/autocross competition street T/A (class E/Sptreet Prepared)... Combined with the 1.24 TBs, Red Konis, 1.25 front, 1.00" rear sway bars, and other misc things (lightening the weight, etc), the car rides and handles great.. cruising highway and autocross are fantastic, as well as recent road course HSAX events lapping sessions, etc.
My leafs are 225 pounds per inch, with a 5" free arch... compressed comes down to nearly flat... ~1"-1.5" (minimal load of ~1/4 tank gas, empty trunk, etc. Before determining what rear spring rate and arch you want (flatter = better handling response) you must know the 4-corner weights of the car, with driver and misc as you'd intend to drive for your peak performance... especially, at least, you ned to know the rear weights. Flex-A-Form worked with me to determine the best setup.. a good company that knows their business! ALL fibeglass mono leafs take a while to settle (mine took about 2-weeks).... have had them since late 1980s.... also, saved about 30-40 pounds vs multi-leaf metal de-arched springs.