attn-nico-ecu-fans-t518224.html
There's the most up to date info on the ECU's. Read that up and let me know if you have any questions.
Regarding the exhaust tubing, I think 2.25" would be good, personally I'm more of a fan of merging it back to a single 3" or something like that.. Honestly, there's no gains to be had by doing an exhaust with any N/A mods compared to the stock exhaust, so the only reason to do it is for the sound. If your stock exhaust is still good, just cut the mufflers out and keep that tubing...
I think the billet torque converters are definitely worth it. About a 2800 stall would be good, because remember the more HP the engine puts out the higher your stall RPM is going to be. You won't get any better MPG with the stall converter, but your off the line performance will be quite a bit better. For G50s, I prefer the high stall TC over 4.08 diff.