IFR is very unlikely (IMHO) for any non ICAO CofA plane - other than possibly night flight which in the UK is technically IFR but VFR-only helicopters routinely fly at night
already.
"IFR" is a hugely emotional 3-letter word in the world of aviation regulation. You can get practically anything approved in the VFR world (eventually) because ultimately VFR traffic can always be told to remain OCAS and this solves most issues at a stroke, but the moment you allow IFR that implies airways traffic and you impinge on the world of "professional pilots", ATC, and all the other guardians of heavenly purity.
Practically, a plane which cannot climb to FL150+ is a bit useless for Eurocontrol routings, and the resulting weather strategies (VMC on top whatever it takes, or serious de-ice kit otherwise).