Junkie, there was a lot of discussions on that in the 705 / 710 thread.

If I remember half of the discussion, the laydown throttle servo does make only very little difference in the CG as it is situated very close to the engine which is much heavier in comparison. 1 or 2 mm difference in the engine height will give you a lot of difference in CG on the other hand.

For the front, one of the heaviest part is the steering servo. Hence laying this down in the front will affect the CG greatly.

I hope I got it correct. If not, hit me in the head !