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?