I rented a non BBP car last weekend...I'm a pretty average driver but do push cars pretty hard through corners on occasion.

Personally, I found there to be a very noticable difference between the two cars. The standard suspension was fine...still fairly sporty and responsive but it was slower to react and heaved more in both the pitch and roll axis under higher g-loads.

That said, the non BBP car was less "jiggly" over bumps and much more comfortable.