oh you were referring to OP not dan.. I thought you were claiming dan's figure of $25k was too high for an 800hp car...
spitfires are a slight improvement over standard coils.. if your standard coils are old and breaking down under load, then splitfires will make a big difference.. but in any circumstance, they shouldnt lose you HP..
In the end, like we've suggested, its likely to be something else (dyno calibration, air temp) rather than your actual car... If your tuner is convinced its the cat, drop the exhaust off on the dyno and see if that makes a massive difference.