I agree D3 would make more sense fluff wise, but you might have to have the no FoF if they have just infiltrated as well, or there would be the same problem. At the minute, nothing can start 12" from the enemy and move more than 6". Hmmm, "Scorpion rush", no, that just sounds wrong
LoL if we give scorps FoF AND Infiltrate. I think we all agree that this would be far to powerful, as the squad would be able to do very significant damage.
I believe that FoF should be changed for ALL infantry. For 4+ armor and exarchs, FoF should be D6. 3+ I think should be D3. As for the scorps being able to get into a first turn assault, anyone can if they start 12" away from someone (except someone with a worse movement than 6"). scorps with FoF, even at D3 with a roll of 1 can just get more into CC. A simple rule change can solve this. Changing the Infiltrate power so that it forces no FoF on the first turn would be quite simple.
On other exarch powers, Defend should either give all the benefits of cover when assaulted (including a cover save, 5+ or 6+), or somehow be applied to the whole squad. Perhaps the squad WS could be increased by +1 or +2 for defencive purposes.
Acrobatic was designed to allow the Howling Banshee Exarch to strike were she wanted in the squad. With the rules change, that power is effectivly usless. It needs to be changed so that the Exarch can choose which models get hit. It should also allow her to be placed in the Close Combat however she wants.
I could go on and on about all the powers, but ill skip on that