...the ability to continue flight after an engine failure at any phase of flight...
For a helicopter to be cleared for IFR (IMC) flight, it must be
certificated in Category A (that is to say it conforms to the certification requirements of CS 27/29). It also has to be
operated in Performance Class 1.
Performance Class 1 operations are those with performance such that, in the event of failure of the critical power unit, the helicopter is
able to land within the rejected take-off distance available or safely continue the flight to an appropriate landing area, depending on when the failure occurs (JAR-OPS 3.480). It goes without saying, of course, that the aircraft must be flown within the WAT limits for the conditions of the day. This definition is not quite what you suggested as you think that continued flight should be possible at all stages of flight.
In the early stages of take-off (up to TDP) and the late stages of landing (after LDP), the aircraft is committed to landing. At all other times it can continue flight or land as appropriate. As the AS355 F1/2 complies with all these criteria, it is approved for SPIFR.
As for maintaining height on one engine, JAR-OPS 3.500 requires a helicopter, when flying out of sight of the surface, to be able to climb at at least 50 fpm OEI at an altitude of at least 1,000 ft above the highest obstacle 10 nm either side of track. If flying at a higher altitude when the engine failure occurs, drift down to MSA is permitted providing the criteria above are met.
Finally, the N models do have better OEI performance.