dont worry about de CG, if you use the full lenght pipe you will need to place your RX battery on the engine box, if you use the stock muffler you will need to place the battery just behind the wing tube, the best options for exaust is the stock muffler or the full lenght pipe, the slimline pitts muffler is heavy and you will loose some power
I personaly dont like carbon fiber props, 23 x 8 will be just fine! I am runing 24 x 8 ( dont use this prop unless your engine is well break in) and pulls hard out of the hover!
here is a video of my plane pulling out of a hover..
http://media.putfile.com/EF-87-yak-hover
[8D]
oohh, and the covering its ultracote