Er, eh? I like sleepers as much as you guys, but:
The E30 was not a sleeper:
1988 M3:
http://www.s14.de/0104/10.jpg
1988 325i:
http://www.blueridgebmw.com/_derived/bmw_3series3.htm_txt_325redside.gif
The E36 was not a sleeper:
1992 M3:
http://www.supercars.net/PicFetch?pic=1995_bmw_m3-1.jpg
1992 328i:
http://www.europeanstarmotors.com/photogallery/DSC01555.JPG
The E46 was not a sleeper:
2002 M3:
Link removed by moderator - please double check your links...
2003 330ci:
http://im1.shutterfly.com/procserv/47b4d605b3127cceb55bcbe0d4930000001610
Granted, it's not Impreza 2.0 to WRX STi differences. No gaylord wing on the back (I should know, I see it every day). But they're not S4 subtle. On every M3, no single body panel has been left unmodified. The engine & entire drivetrain is entirely different. Yes, you can get SMG on a 330i/Ci, but the gearbox is different. Suspension components & layout are different. Interior is different. It's a different car. Period.
Even Mercedes creates better sleepers, thanks to the AMG bodykit being available on an E320. I think the term "sleeper" is tossed around way too much, but I'm sure we have the import tunerz scene to thank for that. Time was, a beat-to-hell-and-back '88 Thunderbird with a blown Windsor was considered a sleeper, not a from-the-factory street racer special.