moving the firewall back on a CG Ultimate..?
I moved the firewall back on mine (Saito 120, same case as 150). It's not a good modification for only your second kit. I changed the cut-outs in the top and bottom lite-ply fuselage pieces. I also added a little right thrust while I was at it. Routing the exhaust was a little tricky too. I mounted my engine down 20 degrees to give the stack more room.
My experience with my current project (second CGM Ultimate) is that you will be nose heavy unless you move you battery behind the lower wing or mount servos in the tail. When I put everything in the locations indicated by the plans, I had to add 4 oz of weight in the tail. I added a hatch behind the wing and moved earthing back to the adjacent former and got it to balance with no lead (including moving the firewall back 1/2"). I do not have the servos in the tail.
I have done some more mods which you can find by doing a search on "CGM Ultimate".
I'm almost done with mine. I'll post pictures soon.
Good luck.