In your position, I probably wouldn't upgrade the battery unless you are going to go with lithium. If you are going to use lithium, mount it in the boot and have a fire extinguisher in your car. Personally I wouldn't want to drive without an extinguisher, just seems logical to carry one anyway.
At the time I posted that I didn't realise a fire extinguisher didn't put them out... Trying to decide where is best for my lithium battery now, don't really fancy an engine bay fire, but then I'm not one for a fire in the cockpit either... Might just tow it behind me ahaAt this stage Im pretty sure fitting the battery in the car isnt an option as I'll be entering in road going production class. Otherwise battery relocation wouldve been quite high on my to do list. Probably wont happen unless I go for modified class
I used a golf trolley battery in my mx5 track car, never had an issue starting even in the winter and weighs about 5kgs and was about 30quid..