Here's my suggestions:
1) I'd add a 4th button to one of the rows so that you can play Neo-Geo games properly.
2) You say your buttons are the exact same layout as Street Fighter but they do look awfully far from the stick. Look at this link as a comparison:
http://www.darkravenwind.com/cdvision/sfcab.htm3) You've got plenty of room in the center. I'd move the 4 way a *little* further up, away from the trackball.
4) No reason to have two fire buttons by the 4-way *and* two mouse buttons. Just make the mouse buttons do double duty. That's what i did with mine and it worked out great.
5) The player two stick looks plenty far away from the trackball. It seems to be about the same distance as mine and i haven't hit my hand yet even during Marble Madness and Golden Tee marathon sessions.
6) Even though you say you don't want a spinner, i'd leave yourself the room in case you want to install one later. Without one you're going to be missing out on a lot of great games (Arkanoid, Tempest, Tron, Discs of Tron, etc). You've got the room, might as well make the most of it.