This thread is a very common one that gets asked around here, your best bet would be to search for it. some proud owners of the 300 2.8 will show off eventually for ya though. its an incredible lens and some day ill afford one for myself.
For what its worth..just get it. Trust us on this one.
This is not the best pic as you asked for, but a test shot I took last month (the 300 2.8 L IS arrived late December) with the 2.0X attached to a 20D at ISO-400 - can't quite remember, but I think I used a monopod
Approx 70% crop