I run an OS 50 with an APC 12.25 x 3.75 and Tower Hobbies .46 muffler. It's a screamer with tons of power. I placed the receiver battery under the canopy cover all the way back. (personal flying opinion here) It was perfect prior to putting the wheel pants on. Now shes just a little nose heavy at most - no other weight added as of yet.
Great combo/airplane!

Good luck

Smitty