I have a friend in the States who flys a Zenair CH701, quite an old one. In the beginning he has a VW derivative, I forget which one, but he has plenty of money so it was undoubtedly a good one. His plane had all the goodies imaginable so was quite heavy and he never got to fly it two up. Incidentally, he also played around with all sorts of propellors of various diameters. The problem seems to be that the cowl is just a tad too big so that if you have a small prop, the engine develops the revs, but the prop is inefficient. If you have a larger prop, the engine doesn't develop the power even though the prop is more efficient.
One day he ran into a heap of rubble when he landed short so he took the VW out. His ultimate aim was to put in a Subaru, but while it was getting prepared, he put in a Rotax 503. Because it was a fair bit lighter than the VW, he had to put about 8 pounds of lead in the front to preserve the C of G. Guess what? The aircraft flew much better with the 503 than it did with the VW.
Not quite end of story.
At about this time last year I had a chance to fly a Subaru powered Zenair CH701, similar to mine. Because it was so heavy it didn't want to lift off until I got to over 50 knots. Mine with a 80 hp 912UL can lift off at just over 35 knots in the same configuration. I don't know how my friend's '701 performs with the Soob, but I suspect its pretty ordinary.
Most of the VW figures I have seen quote fuel consumptions of around 12 litres per hour. This equates to around 40 HP which might be OK for Corby Starlets, but is probably a bit on the light side for a Karatoo. I supect that a Jabiru engine would also have the same problem as a VW in front of a cowl the size of a Karatoo, whereas the 912 has a gearbox, so you can put a decent size prop in front to it.
I had a good look at the Karatoo about 15 years ago, but at that time, there were no suitable 4 stroke engines available. Nice aeroplane though. Andy Shea down at Yarram has a Karaone. First flight was with a half VW. Not good, rate of climb very slow, looked for gap in hedge about half mile upwind. Now has 1600 Jabiru, much better.
Happy flying,
David