03 Ba Xr6T Gearbox And Low End Torque Questions

[XxAcidBurn21xX]

First off a big hello to everyone here! Ive just signed up 2 the forums and am loving it already!

Ok abit about my beast, I have a 4sp auto 03 BA xr6t.

List of it mods are:

*Full Stainless Xforce exhaust system 4" Dumpy into hiflow cat into a 3.5"inch thru to the muffler into dual 2.5" tips.

*Growler Cold Air Intake.

*Tranny Cooler.

*Reconditioned Stock Gearbox.

*Custom Dyno Tune but is still running stock boost and the dyno sheets show 270rwkw.

I had the gearbox sent to Melbourne 2 years ago when I started my build to have it fully recon'd and a 3500rpm stall converter installed. Long story short the larger stally never made it into the box :S which I was furious about. Neways over the last 2 years the car has started shifting rough at the lower half of the rev range. I replaced the rear center bush in the diff with a nulon 1 as the old 1 had a crack in it and I gave the tranny a new filter and fluid but it didnt fix my problem. So I think its about time I upgraded the transmission as I have heard pretty average things about these particular gearboxes. Any transmission builds and opinions would be greatly appreciated

Also Im looking to gain abit more low end torque as my launches suck without a good stall converter, would new valve springs and injectors help much?

Thanks guys hope 2 c u all out there!