Not had a conversion, but I have a 914 turbo in my Europa and am really happy with it, nice to have that extra boost for a short take off run. Though of course it does end up heavier than the 912S 100hp.
This conversion kit seems like a reasonable idea, but they do recommend having a full condition inspection before adding the turbo, and the $8500 is just for the kit, installation/inspection is another $2500 + parts. I guess also the warranty would also just apply to the kit parts.
If you can't sell the 912 I guess it would seem reasonable?