Ok
I'm thinking about the racing new season - yeah someway off I know

but I was considering my options/upgrades to my tele lens.
It will be for motor sport - drag racing where your reasonably close to the action and circuit/sprint courses were sometimes your not........you know the score...
At the moment I have the capable Sigma 70 - 300 and the Nikon 18 - 200VR as well as the 18-55 kit lens.
I don't really want to get rid of the 18-200VR, so I therefore see the upgrade being on the Sigma, and I need more range sometimes..........
What happens if I use a teleconvertor on the Sigma?? Can I? Are the results worth the ££? Could I use the same teleconvertor on the Nikon lens?
What would be my other options? The Nikon 70-200 f2.8? But I'd still struggle with range and how could I justify the 18-200 then?
Any other options - or do I have a reasonable setup?
Sorry for all the questions in one mail.....

I don't mind spending the cash if I can get the good results - obviously I don't have a bottomless pit either. You see my dilemma??
Cheers
Dave