to be totally honest, I really don't notice any real difference between floor mounted and firewall mounted. All of my DD stuff is always firewall mounted minus the Porsches that I have owned and the race car is clearly floor mounted but I have to say I never find myself thinking "wow these pedals feel weird"
I would imagine you would certainly feel a difference if you had a lot of travel in the pedals but if you have enough travel to notice whether your ankle is bending forward (floor mount) or backward (firewall mount) then you have done something wrong IMO.