My car lost RWHP with a stall versus stock converter but it depends on a lot of factors.
1) Diameter of aftermarket torque converter (ie: 11, 10, 9, 8 inch)
2) stall speed
3) STR (torque ratio)
There are converters out there that are MORE efficient than stock so it all depends on what you started with and where you are going.
I had a Yank TP4200 stall that flashed to 5000rpm on the dyno and made 460RWTQ and only 390RWHP in my formula when I had a mild heads/cam. Car ran 11.30s @ 122mph with that combo at 3600 pound raceweight. I could ignite the tires punching the gas at 60mph. It wasn't all that streetable but it was a beast at the track with 4.10 gears
Cheers,
Chris