If you can afford the Canon 2.8 then I'd say go for it. I have the EF 70-200 f/2.8L IS (not MkII) and it's great.
If you want something a lot lighter and can live without the extra 2/3 stop, get the f/4 IS.
don't recommend using the 1.4x converter with either lens. If you want to do that, get a prime. Just MHO.
EOS 7D + grip, EOS 40D + grip, EOS 10D + grip
EF-S 10-22mm f:3.5-4.5, EF 17-40mm f:4L, EF-S 60mm f:2.8 Macro, EF 70-200mm f:2.8L IS, EF 300mm f:2.8L, EF 500mm f:4L IS, EF 1.4x Converter II
2x Speedlite 550EX