I installed the vette servo on my 2002 Tahoe z71 with 200k a while ago.
It tightened up the shifts (didn't do anything to gas mileage). The only thing I THINK it changed (other than making the shifts solid) was that it doesn't do much engine braking in drive. I never drove it in the mountains prior to the install but it seems like there should be more resistance. Even just straight line if you let off the gas, it won't slow much. But I would definitely install it again as the transmission seems much better.