I also do second gear launches. As Pat said some trucks like it, some trucks don't. I made 100+ passes last year launching in 2nd and have noticed no ill-affects to my converter or trans. I'm not at all saying that it isn't harder on your converter - there's no way that it couldn't be harder on it. Try to limit your staging time to limit the heat. I launched between 2400-2800 RPM depending on the track and was cutting 1.66 60' times on 285 BFG ATs and 1.5X 60' times on MT ET cheater slicks.
The 2WD LMM we just put on the track seemed to like the 2nd gear launch and it helped him hold traction better. He cut some 1.9 60' times in a fully loaded crew cab with no spring clamps or anything, full weight including: spare tire, tailgate, hard tonneau, sub and amp, bumper, hitch, bed rug, a tool box in the bed, and 22s on the front. Not too bad since I see lighter 4WD guys that cut worse 60's.