Unfortunately few camera stores have this sort of gear in stock for you (or me) to try out!
If you lived near South Wales I would say give me a ring and you can have a play to see what you think. Having said that if you frequent local birding/wildlife spots then others may be willing to help? Most are pretty good in my neck of the woods. Some photographers are overly precious about their gear - others are happy to share, I am in the latter camp.
The performance of the 300 F2.8L IS with the 2 x extender is very good - but it does need decent light to give it's best.
Incidentally don't go "Green" with envy. My income is so low that I don't even pay Income Tax! A little careful budgeting and it is quite surprising what toys one can afford


