I would say give yourself the option of seeing what the 981 has to offer, but I'm a bit split on answering your question, Ghost.
One one hand, I say ~250hp is plenty for the street, and is a nice spot to land for a car that doesn't weight 4-5000lbs. Our cars are right around 3,000-ish, and regardless of what type of transmission is doing the work, the car's going to move. You'll have your civility and liveability for everyday driving, and enough grunt to get things moving when you need to overtake on the highway or when you want to get into some aggressive backroads.
On the other hand, as you've expressed that this is your last P-car purchase, the other side of me says if you're gonna go, might as well go big. A Boxster S, again regardless of what's driving it, is going to be a blast, and if you felt like spoiling/indulging yourself, why not go for it? For that matter, you could set your eyes on a Spyder if you don't mind literally having a rag for a top.
From what I've seen of your thought process, you have found a way to set up a base with options you'd like to have to where you'll be quite happy with it, and ring in under or at the price of an S. To do the same to an S would likely land you in higher cost territory than you would want. Anyone's going to feel a 50+hp difference unless they have no senses, but if the majority of your driving isn't going to make use of that potential, as many of the owners of garage queen Porsches will not, then go for a base with the options you want and be satisfied.
Besides, when gas hits $6-$7/gal, you'll be doing the slightest bit better at the pump than the guy with an S.