Ian...my combo is a 572" pump gas hemi (800+ hp N/A), 727 trans (footbraking), 4.10 Dana w/ Cal-Tracs and drag radials. The car weighs about 3900 lbs.
The car is set up to run the 1/4. Best N/A pass is 6.05 at 112.9 mph in the 1/8 and 9.54 at 140.4 mph in the 1/4. That was in good air.
best on spray so far is the 8.95 at 149.7 and 5.71 at 121.6 in the 1/8. The air was not too great when I ran that night, but not sure it matters a whole lot when using nitrous.

The converter seems to be on the tight side. I had my converter guy build me one specifically for nitrous, 9" spragless. Seems to have around 6% slip up top. Surprisingly, the 4.10 gear seems to be OK through the 1/4 on the nitrous. Crosses the strip at about 7400.
I'm hoping the switch from the -4 feed to the bigger -6 will help somewhat. Bottle's in the trunk so there's 14 feet of feed line. That much -4 line has to be restrictive?

I have not tried less than 3 tenths delay on the hit. I had it nearly go on the bumper the first night I ran it w/ the new D/A shocks. Got them to where the car is leaving good now w/ the 3 tenth delay. I just have a digiset, no progressive.