Nice thread. I too am interested in various engine swaps for the 86; though I like keeping my car "pure" and as mod-free and stock as possible (only modding things that matter most - suspension, tires, brakes, weight). I also love Porsches (who doesn't
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I think the two biggest things you need to consider in the real-world if actually doing or owning one of these is maintanence and replacement parts and also legality. Primarily - what if the engine, transmission or other high expense part breaks/blows.
LSx 86:
Pros - if the engine blows, it is only $1-2k for a junkyard LS1 or $6k for a brand new crate LS3 (which is still very cheap compared to a German motor).
Cons - if you live in California, it will be extremely difficult (maybe impossible) getting it BAR approved, passed and being smog legal. Rendering doing an LSx swap 86 useless and NOT possible for a California DD and can only be used as a track toy.
Cayman:
Pros - no need to worry about BAR/smog obviously since it's factory and stock. If it can smoke a previous base 911's on the track and better than previous GT3s with an LSD and with very little suspension/engine mods (ie: only upgrading simple coilovers, a tune, exh, etc), then I am sure it'll smoke a garage LSx 86 build.
Cons - if you blow the engine, you are fcked. 3 options if this happens - Junkyard engine will be $6-8k (if you're lucky to find one in good condition and for cheap), rebuild will be $10-18k, or a crate engine will be around $25k.
If it was me...I'd get the Cayman on CPO warranty. Simply and mainly because it would be totally street-legal and I could actually drive it on public streets daily. LSx 86 would basically be a giant, expensive, $50k go-kart in California..and is totally not possible to be a DD.