For the Tahoe in my signature (OBS), I just recently had my gears swapped from 3.42 to 3.90 (factory tire height is 29", I'm currently running 30.5" tires). The low rpm TQ addition is certainly a nice improvement, and around town economy has improved. Haven't had it on the highway/interstate enough yet to get a feel for higher speed economy...my gut feel is that as long as you keep the rpms within a reasonable range, economy will be about the same. Admittedly, my gearing needed to be adjusted for my tires (30.5" matches well with a 3.73 gear set). With the 3.90 gears, my speedo is off by about 10% (I'm now running 10% slower than speedo reading).
As bottomline2000 stated, the 4L8xE transmission is considerably heavier and has a bigger torque converter, both of which rob lots of TQ/HP. If you were looking for a towing setup, I'd certainly recommend the 4L8xE converter as it's more durable relative to the 4L6xE. To convert from the 4L6xE to 4L8xE requires a lot of changes - computer programming, harness, driveshaft length, yoke and pinion changes, and rear end changes...if you're trying to head off "weak link" propagation.
