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.