New good converter worth it?

I got my transmission (904) and torque converter from a guy for super cheap, like $150. He claimed the transmission was rebuilt and the converter is a ~2500 stall TCI unit. I've been driving the car with the new stuff for a little over a year now and while the transmission is mostly fine (flares up when downshifting from 3rd to 2nd at high speed for some reason) the converter slips a lot all the time. I've heard not-so-great things about TCI so I'm not surprised if it is crap but how much extra performance will I get from a nice new converter like PTC? I have a 10.5:1 360 with Magnum heads and small Voodoo cam so it has tons of low-end torque. Also still the stock 2.76:1 rear, hoping to have a 3.91 8 3/4 in the future. My gas mileage sucks at 14 avg. which is way less than I know it can do (only managed 17 on a long trip).