I have not seen any data on the efficiency matter yet - at least not a side to side comparison on an efoil. My main concern is safety - especially once I think about letting someone else use my board - this overrules efficiency in any case.
There are products on the market making it work - so I should be able as well.
Also I would go the minimalist-foil-assist route - just enough to get me on foil and a few 100 meters to the next wave.
Here you can see the cross-section of the impeller - 3mm at the thickest part. At the moment it is plenty strong although relatively flexible and reasonably cheap to be printed at 2,28€.
At this price I can afford many test setups to find the optimum rotor by trial and error - best case on a test bench. Same for the stator 
I have one more point where I could use some advise.
Currently I use these shaft seals in the PTFE or NBR material.
This leads to excessive heat built up in the shaft. Is this problem ignorable since cooling is good in the water anyway or should I worry about quick wear? Please keep in mind this is not meant as 100% efoil but assist only.
Alternatives would be:
- (ceramic) sliding shaft seal as with most waterproof motors
- Labyrinth seal
I excpect all the other traditional boating seals to have even more friction/ thermal issues right? Even more at 800 KV 52 V