a search on goggle will give you all kinds of articles on the " fix " I would guess i read 2 dozen of them BEFORE I got this 99 I have now. A few of them will focus on ONE car, and show before DYNO numbers + Track numbers, then after the FIX numbers dyno/track. Mine had the fix, I know because I called the 800 SVT number to verify the fix had been done. I did this and then later that same week took it to Wayne's Dyno in Jacksonville NC for the STOCK baseline run..... 272hp and 280tq, with the loss thru the drivetrain I'm right at the 320 HP rating. I never came across a Cobra without the fix, but from all the things I have read they were in the neighborhood of being 30+hp down from the advertised power production.
Just for reference the Mach 4 valves make a little more power than the 99 and 01 Cobra 4 valves, and quite a bit more tq. Again do a search you will get all kinds of good info