as far as the OP goes no thats not overkill. my entire floor front to back is a mimumim of 1 layer full coverage damp pro with spectrum covering it all. Overkill is my 6 layers of damp pro in the front wheel wheels (not including the layer on the inside)
as far as the whole SPL deal goes every car is different. flex does hurt scores to some extent. but if you stopped all the flex in the car then most notice a decrease in spl. ive herd some usaci guys say they will only put it on the pass door bc the mic and that increases the score.... really what u need to do is buy extra. burp it put on once pice burp again and so forth until u figure out what works for ur vehicle. if it drops the score then yank it and throw it in the trash (depending on how good u clean it may or may not still be usable)