As ever its all about balance, if you are going to improve/mod the engine then good sense would suggest you need to do something with the other "systems" even if that means just having them checked over!
The problem with 312's is not disc size nor pad size, its the combination and interaction of the pad/disc/caliper that ultimately leads to problems. 312 calipers are thin walled with a large surface area which leads to heat build up, OEM pads depending on supplier are normally classified as being suitable for the lower range of temps that regulation 90 provides for, 312 discs are 25mm thick which dont have much mass to absorb much heat. The general rule of thumb for disc thickness is that for every 10% increase in disc thickness you can get 40% more energy into the disc for a given temp increase.
Having spouted all that BS, it sounds like you've hit the nail on the head in that the shoe needs to fit the foot and you need a BBK like you need a hole in the head.
Might be an idea to budget for a S3/R32 setup more for peace of mind than outright performance.
The B5 RS4 setup is relatively good value, its Iron has large discs which does increase brake torque substantially and is a great solution for those with LM style lipped or multi piece wheels which dont lend themselves to bigger calipers.
Compare the 312's on your golf to say a "standard" 996 Carrera 2
Porsche Disc size 318mm (Dont forget diameter counts for nothing its radius that matters)
Porsche Pad is smaller (bigger pads dont make you stop better)
So why do Porsche stop very well?......its all about balance, they have multi piston rear calipers as well as fronts which have better "feel" characteristics than single piston setups liek your 312's. Single piston setups are cheap to produce and straightforward to make effective they dont lend themselves to good comms when approaching the limit all of which has no interest to the OP!
Found myself a guinea pig for Boxster fronts on Mk5 312's so we'll run em for 1k miles and report back, it worked well on the Mk4 with the same hydraulic numbers and should work better on the Mk5 as the rears being bigger will give a little more rear bias.
Should be a lightweight ally caliper solution without going to the frankly podgy 345mm disc with good clubman pad options for road and track that'll fit under 17's. Couple of months away though. Using 312's discs however it wont suit the biggest is best brigade
Back to the OP good for you for not getting roped into the BBK treadmill but imho you'll need to do something with the fronts because if the power is there and the road looks like fun you'll use it and might just find yourself wishing you had made some provision for some slightly better stoppers
