Will this clear my exhaust tips on stock suspension?

The diffuser is engineered for quad exhaust models, so tip clearance depends on your exhaust system's height and routing. Check that your current quad-tip configuration sits at or above the diffuser plane before installation.

Does this diffuser provide functional downforce or is it cosmetic?

This is a styling piece. The grid pattern enhances the visual presence of the rear end; any aerodynamic effect is secondary to the aesthetic upgrade.

What preparation is required before painting?

Sand and prep the surface according to your chosen paint system's requirements, then apply primer and color coat of your preference. FRP responds well to standard automotive paint workflows.