with a mid 23x/23x cam and a 3200 stall or so, what kinda gas mileage is expected out of a 4L60E? say with 3.42s out back.

 

With a lockup stall, you shouldn't see too much of a decrease in highway mileage vs stock converter.
City will drop a little though from extra RPM at stop and go.

FWIW, I run with a H/C 4000 Stall 3.73 setup (unlocked below 62mph) and get 14cty/19hwy
But then again, I drive like an *** LOL

With my old 3200 with stock lockup (same cam and rear end) I was doing somewhat better at 15cty/19.5hwy
Again, I drive like an ***.

 

With a 2800 stall (with lockup) and a hot cam my daily driver gets on average 16.5 -17 with a mix of city/highway, traffic and spirited driving. On trips I have seen about 23-24.5 Staying in the city I get around 13-14. That's with stock rear gears..