Would have around 3 hours to spend in FCT. Get a snack, some drinks, shower, Cigar room and get driven to the plane. Its the best LH has to offer in terms of ground service. All the things FCT is known for. But I don't know how much better it is then MUC considering that it would be a 2 hour shlep home instead of a 15 min drive from JFK.
I think it's a lot harder to decide if you really appreciate those things. For me, the food (even a KSML) would be meat and inedible, I don't drink or smoke, and don't feel the need just to shower if I'm only coming from VIE and could have showered there before leaving. For me the only cool think is the car to plane transfer, which is not worth a 2 hours commute, even if there's no traffic (public transit) and especially if there is (driving).
OTOH, Assuming LH F isn't something you'd be flying anytime soon, and assuming it's the whole experience what matters (which it sounds like it is for you), I'd go for it. The day after you arrive home you won't even be thinking about the 2 hours. It's a small price to pay in that case.
Sorry if I'm confusing you.