RE: Phase 3 EF - 16
I've never used an ICE ESC so, I'm not familiar with the weight. However, I would put it where it affects the CG the least. The factory ESC "pocket" appears to be very near the CG anyway so, I'd place it in the stock position along with the motor/fan, receiver, battery, etc.and see how she balances. Move things around until you get it to balance correctly.
BTW ....balance it upside down and you'll far fewer headaches with CG.
RJ