tamron lenses for sports

i have had a look at www.ajpurdy.co.uk selection of lenses. Most of it is L
glass costing £900 upwards. I currently have the 28-105 usm and the 70-300.
I seem to have a bit too much depth of field with the 70-300 when shooting
football. even at 5.6. I am currently using sports mode with my 300D as
this is the only way of acquiring servo focusing and continous shooting and
high speed shutter and higher iso setting. Some have suggested that i get
faster glass that would improve the focusing than buying the 20D. Faster
glass in canons own seems out of my price range. I am not a big fan of
sigma. What do you guys think of tamron?