A remap on its own (stage 1 as its called) will net you 40bhp as a rough guide. Compliment that with a performance air filter (Pipercross or K&N for example), a stainless steel cat-back exhaust and a revised diverter valve (common to fail so worth checking/replacing) along with a custom map could see up to 60bhp increase. With the extra bits,its often referred to as stage 1+ (like my own GTI).
Don't worry too much about intercoolers and full induction kits until you're going stage 2, which is a bit more of a grey area...
Hope you got your audio sorted. If not, having done a swap myself, be gentle with the dash trim and keep all screws safe!