"There is no difference between separation in VFR or IFR it is only who is providing the separation that counts."
OK - in that case take 'IFR separation' to mean 'separation from all other aircraft to the same extent as is currently available from RAS- irrespective of flight conditions or Class of airspace. 3nm between identified aircraft or 5nm from unindentified aircraft unless the height difference is known to be > 1000 ft '. To achieve that, all aircraft would need height encoding transponders. It would make little sense for anyone who doesn't yet have a Mode C transponder to fit one - but it would equally make no sense to require that perfectly good existing Mode C transponders in GA aircraft are thrown away.
Only a complete idiot would fly in IMC outside controlled airspace without a radar service - I remain resolutely convinced of that. Hence 100% 24/7 radar cover above MSA would be required to provide adequate safety for such activity. The suicidal pilot choosing to rely on the non-radar big sky theory won't be a threat to a TCAS-equipped airliner if his aircraft is fitted with a height encoding transponder - but he will be a threat to his similarly daft brother coming the other way who doesnt have TCAS.
IO540 - do you really fly in IMC outside controlled airspace without a RIS or RAS?