03 Cobra mods

With an offroad pipe, catback, CAI, 2.8 pulley and a good tune, you can expect to see about 450 hp. You can get another few HP by changing to something like a Denso IT20 plug, or so I've heard. Those are the cheapest mods I know of. After that, you have to start spending real money. :)

As far as the ET, I would guess low 12s with a good driver? Maybe even high 11s, who knows? MM&FF ran a 12.6 or 12.7 with a stock Cobra, so guesstimate from there.
 
biggest problem with ET and more HP will be the IRS! believe me I know as I have ripped one out with 532 HP. Be prepared to spend a lot on strengthening the rear end. I have done it all and now faced with replacing the half shafts...never ending!