I don't think you need extra security in the shape of an immobiliser as they need the key to drive it away or will just lift it onto a flat bed if they want to take it?
Extra security for me would look like a tracker (but then remember that as tracker systems are not advertised on the car it is only a post-theft advantage to get your car recovered - it won't stop the theft). Secondly, security film if you want to prevent 'smash and grab' with you in the car or when it is parked up.
Most important I think is a good quality insurance policy that covers you with all mods declared etc. as well as GAP insurance so that if it does go, you know you are covered