The Tamron 200-500 is a superb lens, I have seen many excellent photo's produced with this lens in both film and digital, in my opinion it knocks the spots off the Bigma!
Incidentally, it can be purchased from Essential Systems in Germany for 793 EURO (£535.60 at today's rate) including vat and carriage. Details at the following web address.
http://www.essential-systems.de/index.php
The Tamron 200-500 works well with a 1.4 Kenco Pro or a Tamron 1.4 teleconverter, I think they're both the same converter with different names! Can't vouch for the Bigma, but doubt it would be very good at the short end of the zoom range.
nirofo.