I put the Iridums in my '01 Cobra and they seemed to work pretty well... but, I replaced them at the same time I swapped the filter with a Denscharger CAI kit. So I can't say for sure how much improvement was due to the plugs, but between the two it sure runs good.
Would I replace the plugs with Iridums again??? Yup, guess I would...
A '97 cobra motor and '01 cobra motor are somewhat different. Besides the '97 was stock...there will be little to no benefits by changing the plugs to Denso's. My brother's '01 is has full exhaust, pullies, wms cold air kit and a dyno tuned Autologic chip....conditions like this will benefit from a colder plug like the Denso IT20.......especially with a motor yours(which is pretty impressive by the way)
I put these in my 01 and dyno-d before and after (with that being the only change) and actually lost 1 hp. I know the guy at the mustang place so it didn't cost me anything. I felt really cool after that but the car does feel a little stronger if that makes any sense...You can get great prices on these plugs on eBay...