Anything obviously wrong with this layout? Enough spacing between buttons, controllers, etc?
Looks like a long reach to the 4-way, but I can't say I see a better solution.
Right now I'm leaning towards Happ Super 8's with a separate 4-way.
Okay, but the drawing says Ultimates for the joysticks. Ultimates are about the least popular Happ Joysticks, and pretty much any common Happ joystick in 4-way mode sucks. Recommmend either a Happ Ms-Pac reunion, the GGG Omni-Stick, or even a Seimitsu for dedicated 4-way stick. The Ultimarc E-Stick (Happ Universal) or J-Stick (Sanwa) would be other options.
I'd like to lose the 4-way and upgrade the Super 8's with Precision 360's, but I'm a bit unclear on the P360's.
Assume you mean Perfect 360's.
Are they easy to wire?
Yes, they have 5 terminals - Up, Down, Left, Right, and GND (not sure of the order). Run one wire to each encoder input and the fifth to Ground. (You need a dedicated encoder, they won't work with the X-Arcade one, and I don't think you can do it with a keyboard hack.)
>Will they perform (almost) as well as separate 8-way's and 4-ways?
No - well depends - if you replace the 8-ways with P360's in 8-way mode and the 4-way with a P360 in 4-way mode, you'll have awesome gameplay. But to switch them from 8-way to 4-way, you have to take them apart and flip an actuator (like a Super). Even then, they are not restricted in 4-way mode (they just don't register diagonals) so the feel is different from a true 4-way stick, but many people like them.