A lot of people (including myself usually) will suggest that you not angle your P3 and P4 joysticks, but instead keeping them parallel to the screen. My 4p Xmen cab has all the joysticks parallel, but I really think this is more personal preference. Just make sure you can get used to the angles before you finalize your CP.
I would also move your spinner a bit to the left if you ever plan to play Golden Tee or any trackball games that require big hits. You don't want to clip your hand on the spinner after your big drive.

Your other concerns are also highly personal preference. Player start buttons on the front of the CP would work fine. I personally don't like them above the player buttons as you have them, and would prefer them at the top of the CP... but that is just me.
As for the 7th button, I am a huge NeoGeo fan, so it is a must for me so I can play them the way they should be played. If you're not a big NeoGeo fighter fan, then it won't make much of a difference to you. You're not actually "adding" a button, you'd just be doubling up on one, so you aren't losing any games if that is your concern.
Also, if you want to avoid clutter (as you mentioned with the player buttons), you might want to consider using a TStick plus for the player 2 joystick. You could then get rid of the dedicated 4way. Just another option.