Ok, my 98 Cobra with a Magnaflow Catback and 4.30 gears put 260 HP to the rear wheels, SAE corrected. Stone stock, it also made 260 RWHP. From what I can tell, the horsepower loss of the 4.30 gears was almost exactly offset from the gain due to the catback.
So you are telling me that a JLT package will bump my car over 30 RWHP? Ain't gonna happen. On an 03/04 Cobra, yes, first gen modular snake, no way.
Also, the graph N2O posted shows a peak RWHP number of only 263. This is a far cry from the 293 RWHP Starman's graph illustrates.
Yes, gears affect RWHP, but probably on the order of several percentage points, not the 10% or more that would be needed to cover a 30 HP spread (based on a 300 HP engine). I feel that I lost 5-10 RWHP by going to 4.30's. The factory Ford catback on the 96-98 Cobras is not very restrictive, so I probably gained back the 5-10 RWHP with the MagnaFlows. I don't have dyno results in between mods, so no proof of my "feelings".
All I can say is Starman has the 98 Cobra King of the Factory Freaks.
Hell, I've read posts of full bolt on 96-98 Snakes barely breaking 300 RWHP with long tubes, off road pipes, custom tune, underdrives, bigger intake tubes, etc.
Here is my factory 98 Cobra dyno run with only the intake silencer tube removed and 22,000 miles on the clock:
One Stock, Run of the Mill, 1998 Mustang Cobra Dyno Sheet