Having had our Europa broken into whilst on our driveway and the Elan stolen & later recovered, I can understand your thinking. Trackers are available over here and were a big thing once but with the advent of companies using them to track employees in company vans there seems to be an industry set up to defeat them. You can find instructions on what to buy and how to defeat a GPS tracker on t'interweb....
http://www.ehow.co.uk/how_5718205_block-gps-tracking.htmlThe Europa was entered by using a screwdriver in the lock and simply wrenching it round. The Elan didn't have a mark on it and the police said the thieves had keys, perhaps not surprising as the types of locks on these cars are pretty poor and I've used the wrong key to open that car myself !
So I go for the battery cut-off with removable key with a couple of additional cut-out switches hidden away. If you have an electric fuel pump that's another handy circuit to bridge.
A lot of the Elise owners fit removable steering wheels for both access and anti-theft and I reckon that's an excellent idea.
I think visual deterrents are the way to go, obvious steering wheel locks (or remove the wheel ! ) and flashing LEDs from either a genuine of fake alarm system are the first line of defense, followed by preventing it from starting should deter the enthusiastic amateur thief. A pro thief will have a low loader and there's not much you can do then.
Brian