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