I've never had a problem with the paint separating form the PEI, although I mostly run the bed at 70C and only occasionally at 110C. I cleaned the PEI with isopropanol before I applied the spray paint.
The problem with using something dark below the PEI is that the PEI sheet is flexible so it needs to be stuck down with adhesive, and the reflection from the adhesive is likely to be strong and not uniform.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
