Mine is currently around 200. After the winter I'm planning a few mods to put me closer to 300. They're a LOT of fun to drive. I'm planning on eventually hitting 400-600hp out of a daily, maybe more. By the time I'm at the 800+ range I think I'll prob take it off the road and put together another 300-400hp 2.3 stang to DD.
I think that 300 or so is a good practical limit for a street driven car and can be accomplished without a great deal of effort. The stock Mustang chassis starts to get unruly when power levels exceed that amount, unless you start improving the chassis.