What are you considering horrible turbo lag, time wise? My truck and a few others I've talked to, that under WOT, you can count to three and BOOM it takes off. Is your lag time worse than that?
What gets me is that the 7.3 PSDs didn't seem to be affected like this. It might also be though that the ambulance I was driving had 5.38 gears, so it was spinning up under much less load (for lack of a better explanation)
I wonder how much better or worse my truck would run if it had 4.10's under it. It seems like with a slightly lower gear it would shift less on hills, particularly at about 50-55 mph. If I get into it anything over 10 PSI, it immediately wants to shift. I can hold the speed, but if I want to gain some on a moderate hill (not strait up and down, but not flat either) It has to downshift. I'd have to think that with an extra 3 or 400 RPM under it, it would be further into the power curve, and therefore less likely to shift.