I've done MGW, stiffened trans mount, and dot4 fluid. My car shifts great on motor now even up to 7300. Stiffening the trans Mount (backyard engineered not whiteline mount) helped a lot. But mine still locks me out of 2nd after a good 60 foot, spraying a 150 shot out of the hole.. Cut some 1.5s on slicks, but every time I did, I'd either have to wait a long time for second to fall into gear or it would lock me out. Ordered my Che torque limiters tonight and will report back how it works. I'm convinced the added power from the nitrous launches is torquing it over more. Causing that shift shaft coming from the remote shifter, to be misaligned going to the transmission.
Did you upgrade your motor mounts? If not your still getting some deflection causing your mgw to mis align and lock you out under load. The remote shifter design requires near perfect alignment under load to cure lock out. The slightest movement can play havoc with your shifting! I recommend these motor mounts which since I've installed them i have not locked out in second and i have no 2nd gear cold nibble:
2011-up GT/BOSS 5.0 Urethane Bullet Motor Mounts - Justin's Performance Center