When mine was first IVA'd, I had the factory fit full weather gear. Richy is right that it would have a riser bar in the middle to lift it a bit to give you plenty of head room. The only thing I didn't like about it was it was fully enclosed, so prone to the windscreen steaming up. I moved to a half hood and that issue is solved.
In heavy rain, on fast roads, you do get a bit of spray in through the the open area at the back. The other area the water gets in is under the door edges. I tend to use towels to keep out the worst but toyed with adding a strip of 25mmx25mm aluminium strip to the side of the car under the edge of the door. This would act as a deflector to the majority of the water. You are then left with a bit of water coming in over the edge of the windscreen but that also happens on £75k Morgan's so not a lot you can do about that.
With the full roof or half hood fitted, getting in and out is a nack, but you soon get used to it, you just have to use a certain technique. You would be welcome to come and have a look at mine and give it a try if you want. I think you have my mobile number already. Give me shout if it would help.
As an aside, I am not sure if SBFS have done a half hood for Cobra Hoops. You might have to have a bar of some sort added to give the shape as the normal half hood uses the roll bar to make the shape. If you you look on their website, it is my car that is featured so you can see what I mean.