8mm sounds alot. Get the floorpan and subframe checked by someone that knows what to look for, by this i mean stressed paint, cracked paint, new vs faded paint lines on the edge of the floor where the cradle/control arms sit, uneven rubber bush settling etc..
I hope im wrong when i say this, but it could be possible that you have either floorpan or subframe damage. I know the suspension/alignment people are saying its fine, all they are usually concerned with is that the wheels track straight, which is still possible regardless of damage. Ive seen a 10mm difference in which the floorpan was damaged and it still aligned straight.
Check that the offset of the rims are all the same, then check if the wheels are aligned all the same. Its possible that when the wheel alignment was done the total toe across the car was acheived by having different settings from left to right, what im saying is dont ever expect the same dimensions from the tyre to guard with every corner, you never will have them identical....however, when I say this, im talking a few mm at most.