New battery for my Subaru Impreza….

Good morning world!
As usual I have been looking into ways in which I can improve the performance under the huge scooped bonnet and inside where the sporty seats reside…

I know I made snide remarks about the £4,000 +vat engine tune by Scooby clinic to increase to 400bhp but I was seriously considering it the other day but then my colleague reminded me about the new supercar-eater Nissan are bringing out next year… I should just save my money till then!
I was also contemplating upgrading the car bulbs by way of a Xenon HID kit, some cars look awesome with nice bright HID bulbs, and they give of a strong clean light. But then I thought that I might have to upgrade the Battery to cope with the extra power that would be drained by the new bulbs, plus it would help if I decided to upgrade the stereo as well I suppose. Would upgrading to a performance car battery improve the overall performance of my car, I mean would I get any more bhp?? I really do not think so…
Does any one know otherwise, as I would be really interested in knowing the answer to this as I get confused when people like Bosch offer “performance car batteries”….. Performance in what sense??