My SE has an MDS .48 with the MarkII muffler on it and I did nothing to balance it. My battery pack was mounted on the little platform right behind the wing tube with the RX just fore of it.
The performance was pretty good. However, I recently moved the battery pack back just behind the radio compartment (cut some covering and built a little box back there) and WOW!!! What a difference! Like a completely new airplane. I never rechecked the balance after I moved it as I knew that the plane was nose heavy enough before that I was worried I would have to move it even FURTHER aft. But I like it there.