Nobody is going to have a solid answer for that question.
The cats on a foxbody are usually restrictive from the get go.
High flow ones are less restrictive, but you may find that they don't last that long.
With a proper setup your h pipe should be good for an easy 500rwhp and up.
Remember the pipes on newer cars are very similar to the 5.0 pipes, in parts and design.
Guys run 03 cobras into the 600rwhp range no problem using 2.5 catted x pipes.
we are talking about how many hp a 5.0 motor can produce before BBK's 2.5" H-pipe with cats will become restrictive.
im guessing i have maybe ~400 engine hp so I think ill be alright flow wise with the cats it sounds especially since the 03-04 cobras can make 500+ hp with the same size pipe with cats. my h-pipe is off the car now I thought id check and see if I was running a restrictive pipe before I reinstall. thanks for the replys.
If you have a turbo, supercharger, blower, etc. it really won't matter (to a certain degree)...but if it is a carb or efi it will make a difference above 450 ponies.