Well, I'll let you know how I find the 190XPRO- it's supposed to be a good compromise between sturdiness and weight- it also has the ability to reverse the center column,
and "fold" over to a boom-type tripod- great stuff for Macro work. Maximum height is about 1700mm, minimum about 110mm.
Hopefully it's going to be there when I get in from work tonight, so I can have a quick play.
The only problem with the Canon Extenders is that they'll only work with the 70-200 f4L, they dont work with the EF-S lenses (of which I have the 17-85mm IS) but then I guess I'd use the 70-200 if I needed longer with that lens anyway, and besides- 400mm @ f8 sounds pretty appealing either way....
Also had a play with the Tamron 90mm 2.8 the other day, first time I've had a chance to give it an airing- it's brilliant!