This is an interesting thread, perhaps I can provide some food for thought. I fly over the sea for a living and have jumped into the sea and used underwater escape trainers many times to practise what could happen during the ditching and how to escape, as well as how to survive once you've escaped. I have also flown on Search operations, so here are a few things that might help.
Firstly I am not convinced having the immersion suit half donned is such a good idea, even if you have tried it in the car. Most immersion suits I know of are difficult to don in an open room let alone the confined space of a cockpit. You don't know what emergency may lead you to a ditching and what actions you need to take as a result. For example, a benign engine failure where you have plenty of height to glide, you might have time to don the suit. However I think the time would be better spent flying the aircraft and doing any drills, either trying to restart or securing the engine if it won't restart. If you had a fire in the cockpit that leads to a ditching, are you going to be in a position to don the suit in this scenario? That's just a couple of examples, I'm sure there are many more if we thought about it. We have a saying where I work that; "it's worth the sweat if you're going to get wet". Although the suit can feel a bit uncomfortable and cumbersome it is worth putting up with it for a few hours versus not having it donned properly and not surviving as a result.
Rather than having to don a suit it would be better to fly the aircraft, do any emergency drills, unlatch/jettison doors/windows, locate where you are and make emergency transmissions. Donning the suit is just a distraction from these important tasks.
The impact may or may not be violent depending on how the aircraft is flown and the sea conditions, but it is likely the inrush of cold water will be quick. You suggested that the liferaft is less important, I would argue that it is the most essential piece of kit you could have.
If you think about the basic survival principles in order of importance;
1. Protection
2. Location
3. Water
4. Food
These apply at sea just as much as they do on land.
Your liferaft will provide Protection and Location. Depending on where you were to ditch in the North Atlantic you could be there for some time awaiting rescue. On the UK side for example we no longer have long range maritime patrol aircraft as result of defence cuts. Despite having an immersion suit on you will at best have 12 hours survival time in the North Atlantic in calm water. If you have a liferaft your survival time will increase rapidly. The other point is that if you are just floating in the water the most that will be visible is your head and shoulders. Having searched for people and small objects of this size in the water you will not see them until you are virtually on top, literally a couple of hundred feet in height and a couple of hundred yards, in calm conditions. The liferaft presents a much bigger target to detect and markedly increases your chance of being visually located. If for some reason you can?t get into the liferaft then make sure all the survivors buddy up. This makes you a bigger target to locate but also will helps morale and trying to keep each other warm.
Make sure you have plenty of fresh water, whilst being absolutely surrounded by water none of it is drinkable. You can get survival sachets of water and they are essential. It is also essential to take sea sickness tablets once on board the liferaft, whether you feel sick or not. If somebody is sick it is likely to start a chain reaction. You don?t want to be sick as it will dehydrate you and deplete your energy. A reverse osmosis pump would also be a good bit of kit. It will generate you fresh drinking water as well as giving survivors a task to concentrate on.
There is a lot more to do and think about once in the raft but I am sure a judicious search on Google will come up with more on this. If not, I am more than happy to answer any questions.
Here is a link to a regulatory document the British Armed Forces use; it has a good couple of pages on survival times in the water at different temperatures with different layers of clothing on. The information is on pages 38 & 39 of 166 of the.pdf
http://www.maa.mod.uk/linkedfiles/regulation/fly2000seriesprint.pdf
To give you an idea of temperatures of where you are flying, this could be useful;
http://www.ospo.noaa.gov/data/sst/contour/global_small.c.gif
Hope this provides some useful food for thought.
Simon