The view at my Training Organisation is that the IMC Rating should NOT be used for the purpose of planned cruising in IMC. However, the Rating is quite adequate for making a climb through cloud under a radar service in order to reach VMC. Similarly, it allows the IMC Rating holder to descend through cloud under a radar service to 'VMC below' or to fly an approach under IFR at destination. To plan an entire flight to operate in IMC is, as RC states, not really what the IMC Rating was intended for.
No, radar service is not available across the entire Atlantic. After leaving UK radar cover, you are on a Procedural Service on an internationally agreed route until you reach radar cover on the other side. Note that it's only 'agreed'; some years ago I was on a pond crossing when we were given the 'heads-up' on Guard from an American voice as 2 Bear Hs crossed our route at right angles on the way to Cuba, no doubt!
There is absolutely no 'requirement' to 'upgrade' to RAS on encountering IMC if you have previously been receiving RIS. However, I am still of the opinion that it is essential to have some type of radar service in IMC; whether RIS, RAS or Radar Control depends upon the circumstances at the time. But pilots should only request the level of service they actually NEED.