Andrew,
Not that my approach to my old 981 Cayman S was right (I did have some evolutions)..buying fitted packaged solutions avoids a lot of hassle but I found the learning very fulfilling.
Brakes, I first upgraded to 991.1 S front brakes which gave me 6 piston (v 4 piston) calipers and slightly larger disks (£1.8k fitted). Then fed up with cleaning all the dust out of the disks' holes I replaced OEM disks with Girodiscs (£2.5k fitting 1.5hrs). I tried various pads and ended up with Ferodo DS1.11 and DS 2500 front and rear (£450 and £270 respectively). I never bothered with brake lines. Brake fluid was Castrol Racing SRF, which at £50 a litre is OK for lots of track days but overkill for road. So if I were doing it again, I would go either straight to Girodiscs on OEM calipers or 991.1 S callipers and Girodiscs, pads being a use directed choice. I also changed the master cylinder for a 991.1 GT3 part (£300 + 2 hours and 2 litres of fluid for a thorough bleed)
Exhaust, I found that a manifold with a low cell count cat will with hard use, destroy the cat and cause MOT emissions failure. As my mate in the USA says, it is a matter of when, not if. My Cargraphics 200 cell manifolds were rebuilt two times over 30k miles and 55 track days. Luckily I live by their factory, have lent them my car for testing, and the cat supplier gives Cargraphics 2 year warranty, otherwise £500 each manifold. I kept the OEM rear section of the sports exhaust system which I assume RPM would do. Cargraphics (there are other brands) manifolds are c£2.8k the pair + fitting.
Centre Radiator - I had one fitted to add extra cooling capacity but mine had a 3.8L engine and would be running on track often at higher than normal ambient temperatures. It is unlikely you would need this.
Engine/tune - you might ask if they fit the larger 3.8L throttle body, also make sure the Cobb map = the hardware upgrades. The BMC air filters (£140 a pair) are fine but you gain only at the very top of the rev range. This is how much debris I got out of one side after 6 months, maybe 6 track days, debris pile is about 25mm square, 5mm high - I change them frequently....
I think if you concentrated on getting more air and fuel into and combustion out of the car you would find a difference in delivery and also power. Exhaust, you can spend £££s on exhausts, but what is the purpose, if for engine efficiency then fine, a decent manifold with merge collector will do that so that is the feature to look for. Intake together with exhaust is where the power gains can be made so IPD + larger throttle body + manifold + tune is where to put your money. I'd forget the brake upgrade and the GT4 front.
Ralph