It depends on what they are replacing. If you are replacing a saleen high flow cat, the gains will be minimal if any. If you are replacing some stock ford cats, then you will see a little gain of like 5-10 rwhp....maybe 15 tops.
I gained about 10 from replacing my cobra cats with an offroad X. Over all I went up 18 hp, but I also received some tune work at the same time, so, it is hard to say how much of that 18 was tune, and how much was the X.