I have now seen a couple of very nice Boxster S's with 3.6 and 3.8 liter engine upgrades, and neither seemed to command a premium over nice original cars.
Here is the the 3.8 liter. Just building the engine would cost that much. The 2000 S with 3.6 liter (LN Nickies on a 3.2, not a 996 engine) was in excellent shape with LOTS of factory options and tasteful upgrades (PSS9 suspension, 19" wheels), yet it went months before it sold for $15k, which is about half of what just the upgrades cost. I drove that car and it was/is something special. That car got me thinking about buying a Boxster, but it was sold when I was in the market.
I guess I wonder why people don't snap these cars up and/or bring a premium? It seems that if you are spending $20-$30k to get a big displacement engine built and installed that it would add value to the cars, especially when they also likely took care of the known issues in the engine. Thoughts?