No extra weight needed with the OS 46, 50, or older style 50 FSR ABC. We're flying two US40s (for dual-ing crow landings and stupid fun) - one with the new OS 50 and the other with the older OS 50 FSR ABC....both run great with unlimited vertical. It sounds like you've made decent mods....I'd beef up the landing gear block as well.

So no extra weight needed...tail or nose with the standard size 2-strokes mentioned above. We do have 2400 mah batteries right infront of the CG, but even with a smaller battery, you should have no problem.

-Juhan