Pesonally... after dinking with shims for years I'm really looking forward to the SPC arms so that's what I would change to.
Even if the frame is powdercoated I'd mask off all the unaffected areas, do the minor clearancing on the gusset, and then mask & repaint only the affected area. It's such a small area it's unlikely anyone will notice the spot-painting especially if it's a somewhat close match.