The only good sleepers I know are domestics
Buick GN
Pontiac GTP
Ford Taurus SHO
GMC Syclone/Typhoon
Spirit R/T
True sleepers tend to be more of an American thing if you're talking stock, of these I know the GTP responds well to mods, and the GN...well, it doesn't really need them
The Syclone/Typhoon have the same engine I believe, I'd imagine you could have fun with a Spirit R/T, they are turbo'd after all, and the SHO you can't do too terribly much with. I know this doesn't give you any suggestions leaning towards imports but I honestly can't think of any, everything's been done to Honda's, so a swap wouldn't catch anybody by surprise. If I had to give you a choice for a "sleeper" I guess I'd have to go Tiburon, I haven't seen too many of those real quick yet, but a shaved badge GTI with different wheels would probably be an excellent choice. Seeing as how they range from everything from VR6 to Diesel power.