I dont believe 4.10's cause a greater powerloss than a numericaly lower gear. Most of the drivetrain loss is in the transmission, or in your case, the IRS, which is significantly less effective at transmitting power than the SN95's live-axle.
As for gear, go no less than 4.10's. They are a great city/highway/track gear b/c the revs are tolerable, first gear is usable, and the pull is sick. The only problem with 4.10's is at the track, you'll need to shift to 4th right before the traps. Many cobra owners i've spoken to recomend 4.30's. These apparently put you well into 4th gear before the traps, so you can "pull" through the finish line. TO me, 4.30's seem like they would be a bit much for the street.
As far as bolt ons, they're worth practicaly nothing, and the HP/dollar ratio is pitiful. A near surefire way to gain 20+rwhp would be longtube headers (BBK, Kooks, Bassani anything but MAC) and matching X-pipe. Catback, as stated, is all about sound, not rwhp. The FR500 is definitely cool, what with the pressure activated muffler valve, but if you're interested in keeping the police at bay, my Pro-Flow mufflers are quiet and behaved in normal driving with a significant growl at WOT. Too quiet for me, but you may like it.
Dino
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96 Cobra - a bit too stock. 14.0 @ 101
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