Sorry to be late to the thread. Some great posts here and quite the education.
I have been interested in the II and SII, on a SP waiver. There was a long thread on PPRUNE on how the waiver is not legal in the EU. For what I got out of it, you can fly into the EU, and then leave, but cannot fly within the EU without a SIC.

I have read a lot about the Clifford and Sierra engine upgrades and they seem like an awesome way to breath some life into an old frame. Sierra also offers a 120 gallon tank that will add about an hour to the I, II, and SII. Of course the gross weight mod would be a given.
Anyone have any experience with the extra tank?

I think the II is a great airplane. In regards to breathing new life into a old airframe. The company I worked for had a 1966 Hawker 1A that they bought new and twenty-five years later put Garrett 731's on it. Super airplane, but the old airframe, old wiring, old avionics, old systems. Those are the things that made the airplane too expensive to operate. The selling price didn't come close to recovering the cost of conversion.
Dave

I think as the used prices fall on early CJ2's and 3's, the FJ-44 mods for the 500/550 will look less attractive.

The advantage Sierra has had, is that they've been competing with the used CJ/CJ1 market. Which don't have the range, nor any available fuel tank mods like the 500 series.