I used to transport a SIB, engine, dive gear and one passenger using a Citroen AX, which was roughly the size of a Ka.
The hull was folded up and went on the roof rack. That particular SIB was about 11 or 12 feet long, so something like 3.5 metres. It was a good boat for 3 or 4 people and could take 3 of us on a reasonably calm sea with dive gear.
At various times, I have considered roof racking an inflated SIB and taken the measurements. For any SIB over "tiny tender with a slatted floor" size, I think it would be dangerous, bad for the boat and awful for your fuel consumption.
The idea of putting a RIB on the roof rack horrifies me. You would probably be stopped by the police for an unsafe load assuming you made it round the first mini roundabout without toppling over.
Assuming you are not going to change your car, a small SIB is your only realistic option, unless you go for the idea of keeping a bigger boat at a marina or in a lock up compound on the waterside.