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.
thanks that gives me a better understanding whats going on with your combo .the next thing i would look at is front end travel and stiffness and the rate of extension on the front ,i have changed mine to keep the front down on the hit and screw the hit earlier.