An H-pipe will only help your bottom end. You do not have to worry about your exhaust hurting you until you make it too free flowing with out changing the induction system. But with stock headers and catback you will be fine.
Are you putting on an off road cat back? If so I'd expect more than 5 to 7 HP. I felt a noticable change when I put mine on. Probably closer to 15 HP.
Your car will sound meaner, but will not be as loud as aftermarket mufflers. I heard a Guy with a 95 who had just cut the Cats out and replaced them with a solid pipe and his car sounded pretty cool. Not near as loud as mine, but it still had that aggressive sound.
Hope that helps.
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1995 Mustang GT