The 03/04 Cobra does have a weak spot and it's the piston lands. Ford had to compromise between strength and emissions. The other problem is heat which these engines make plenty of!
Although many claim 1000HP on the block is capable, the stock pistons won't last very long at that level even with the best tune in the world.
Personally I think 450RWHP is the magic mark myself for a daily driver, of course this is just my opinion too! It all depends on what you're going to do with the car, it's current state of tune, and how you drive it. You can have a 500+RWHP Cobra that will last forever as long as you don't beat the living crap out of it day after day. The first time you take it out on the highway and try a 5 mile long 160MPH blast you'll see how long it lasts though! Many a terminator has bit the dust during extended high speed runs.
U.M.