First question:
What year is your Cobra and what mods do you currently have?
As far as gains per mod go, its just way too hard to take a number and start adding things up.... a dyno is pretty much the only way to get definate before\after stats on any setup.
You have to help me... what gear ratio comes on your Cobra from the factory? Some Cobra guys may want to shed some light on what they think the best ratio is for street\strip duty.
In most cases, gears are one of the best bang for the buck mods there are...
Some other things to look at:
An 80mm C&L mass air kit... (Comes with a K&N filter)
A dyno tuned chip with multiple selections... (One for a base tune and several other more radical configurations when that Camaro comes a calling)
I recommend a JMS tuned Autologic chip...
www.jmschip.com
For exhaust, I'd go with a Bassani X-pipe and just about any cat-back system you like depending on what pleases your ear. (I lean to Dynomax)
Other than that, a small shot of nitrous wouldn't hurt....
