TB/Envoy I6 converter is 2400rpm. That's behind the I6... stick it behind the V8 and it's a little higher due to the new power input.
A 3000rpm stall converter built for 3-400hp will be considerably higher behind 600hp. That's not necessarily a good thing if it wasn't designed for it since the case is probably ballooning under that much stress, increasing the internal clearances around the turbine making it stall higher.
The high stalls from reputable companies use stronger cases and internal parts for high hp levels. Back in the day it was common practice to install a S10 converter behind worked 350s since it was a cheap mod that gave a lot of stall. Some found out the hard way that those things would go BOOM and leave a huge mess if you pushed it too far.
Back on topic: the TB converter will be a good "mild" performance converter behind a healthy V8. Good stall and won't feel too loose. Go drive a I6 trailvoy to get an idea of how they feel. As for towing, I believe they use the same clutch as the V8s so that shouldn't be a concern as long as you have a larger trans cooler to make up for the added slippage under heavy loads. I personally wouldn't take it beyond 500hp though. At that point you'll be needing a built trans and a good converter will be a great idea (Yank, etc).