Some topics. I kept the list fairly loose, because there is no point knowing about just one aspect when it's all interelated:
Cars. Engines. Exhausts. Fuelling. Combustion. Driving. Maths. Physics. Engineering. Vehicle Dynamics (a subset of physics I suppose).
You will then discover that the modifications you are planning won't make a jolt of difference to the actual performance of your car, but will make it feel faster due to the fact that a) you want it to be having spend silly money on them and b) they'll make more noise, so you think it's quicker and c) you have to fill it up with fuel more often, making you think you must have more power.
The only chips you need to buy will come from Ebuyer (or similar) or a fish and chip shop. The only mapping you need is an AA Road Atlas. As mentioned before, turbo diesels can benefit, in performance terms, from chipping/mapping, but you lose out elsewhere (economy, emissions, engine life etc). But as diesels are pathetically weak anyway, it's not asking much to improve them