about a year ago I found a gas cap and water impeller for the Suzuki 2.5 at
http://www.boats.net/ A friend converted a Honda about 8 hp to an xlong shaft - somewhat similar. He not only had to get the extension sleeve but also replace the drive shaft and the shift linkage. You may not have to change the shift linkage on the 2.5.. dont know for sure.
I have the short shaft and have never felt at all like I needed the longer shaft either solo or with two people. I sit solo in rear and the outboard is mounted just behind. Curious why you would want the longer shaft? Most small sailboats do work better with a long shaft but I dont think the TI needs it.