I went from 3.73 to 4.10 and the difference is nice, gas mileage even went up. I do not run N2O anymore, but with a 28" slick you can run on 4.10's. 3.73's top out at 114mph in 4th.
I've seen what just adding Stage 2 cams can do. VERY IMPRESSIVE! Kurgan ran consistent 12.80's before them. Yesterday at Bradenton (45*) he ran a 12.24@110! SO that's my vote.
http://www.modulardepot.com/product_info.php?cPath=23_27_28&products_id=30
These are stage 2 cams ready for ported heads. Kurgan gained 4-5/10 in the 1/4 with them in stock heads. You will need Comp Springs and a tune.
Sure the Steeda piece is nice. I'd rather save some money and go with UPR though. But the filter Steeda sells does not match up to a Pro-M without some type of adapter plate.
I went from the dealer to Meineke and had them welded, now they are matched up with MAC LT and O/R X and very loud. IF there was any loss in power, the sound makes up for it.