I'm with you on the 6 speed auto swap. They have a very high torque capacity stock (look at the HP new Corvettes and Camaros are making). So unless you go 1,000 hp turbo motor swap you should be a happy camper. This is a swap a few TBSS guys have done. I cannot remember the name of the company, but some google searching should find it. They specialize in these swaps. I contacted them prior to even ordering my 355. They think the swap is very doable in the colorado platform. But they did mention the TC issues. I'm surprised to hear the full size TC would be a problem...I wouldn't be doing it expecting it to pay off. Just looking at future options (built 4L60, upgraded to 65/70e, 4L80e, or 6L80e.
It may not "pay for itself", but what mod does? It would be quicker, more fuel efficient and just plain cooler than a 4L80E swap.
This talk about going 4L80E makes me think of when people tried to talk people out of 700R4 swaps back in the day (or old school SBCs over LS swaps). I get tried and true is good. But at some point we need to move forward.