I think the difference is that whereas an IR holder will plan a flight through IMC, MOST low hour IMC rating holders will probably not PLAN to fly through IMC unless its fairly gentle [ie. marine layer type stuff, where you climb to VFR ontop]. I'm sure I'm about to get shot down by this statement, and I'm sure there are many very expereienced high hour IMC holders out there who [quite rightly IMHO] do treat it like an mini IR, but I believe this is a reason why you can't compare IR and IMC accident stats.
There is a danger in using the IMC rating like an IR though, and that is that with an IMC rating you're responsible for negotiating you own CAS crossing clearances, radar services, DACS etc. I'm used to flying the US IFR system, where you are cleared before the flight [ie. you get your route clearance before take-off: say SLI dct V64 V363 OCN dct etc]. Any CAS en-route disappears as I have been pre-cleared through it, they may amend my route during the flight, but it is unlikely and if they do I'll get plenty of warning or vectors. Likewise, if I file a route they don't want me to fly, then they'll issue a new route which is ok as you know before you take off what your route is going to be.
Now imagine you have planned an IFR cross country in the UK using an IMC rating, you take off and expect to fly SAM dct CPT dct DTY dct Nottingham. You negotiate with Solent who are kind enough to let you in, so you intercept and fly OB on the SAM 010R to CPT. After CPT you reach some bad weather and you're now in solid IMC, so you call Brize Radar for a Radar advisory service, and you get the reply "sorry, too busy, FIS". So now you're in IMC, with no radar service and a little later you're approaching East Midlands CAS. You request a crossing service and they reply, "not cleared into the zone". Now what? You're in IMC, you haven't yet managed to pick up a radar service and you have to re-route around E Mids CAS avoiding any bad places to be [parachute jumping for example....unlikely in IMC but you never know].....and all this single pilot. And all this can be done after 15hrs instrument training....makes me shudder. As an instructor friend said to me yesterday, "a newly qualifed IMC rating holder is allowed to fly around in cloud and bumble their way through an instrument letdown at the end of the flight"
I'm not sure if there would be any difference if you pre-filed an IFR flight plan for this route, it may mean that you're MORE LIKELY to be allowed to cross CAS, but there is nothing guaranteed, and as one instructor said to me "you don't need to pre-file and I wouldn't as you could be subject to long delays".
Now wouldn't it be good if we had a network of low altitude airways, specifically designed for the IMC holder, providing automatic clearance through CAS and a radar service.....oh well.....
Cheers
EA
Note: I'm not slagging off IMC holders at all [me included
], and I don't think the IMC is a bad rating. No doubt there are many highly experienced IMC holders out there who p*ss all over instrument rating holders, so don't take this post the wrong way.