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The reason you don't need a BBK at the rear is that with greater front braking from a front BBK, you'll get more weight transfer to the front - and away from the rear - during hard braking. This reduces your capacity for braking at the rear, since friction force (between tyre and road) is proportional to normal force (vertical load), and you've reduced the vertical load. Up to a point that's not a problem, as long as the car is still limited by front tyre grip under braking you're all good. If you then add bigger brakes at the rear, you'll be moving the rears closer to locking again (in reality, triggering ABS). Contrary to what you might expect, a rear BBK will likely increase your braking distances because you'll become limited by your rear tyre grip and those tyres already had less grip than the front tyres. You'll also increase your chance of binning it on track if you try and trail brake!

Why have you decided to replace? Do you have planned particular usage that stock doesn't do well? Done right, upgrade to BBK is not cheap, without actual need (if for example you only daily drive yours) you won't experience pros of BBK (mostly higher heat capacity/better cooling rate) but will have to suffice cons (except obvious spending a lot of money for naught, also reduced compatible wheel selection, sometimes requiring buying new wheels and tires, possibly narrower compatible pads selection, possibly less resilency for corrosion issues if driven throughout winter with roadsalt and it's race caliper w/o dustboots & dual-piece rotors, sometimes extra noise issues). Done wrong (where i write off retrofitting used calipers from other cars with different layout/weight distribution/master cylinder ratios, considering only mechanical fitment and cheapness) may make braking even worse, introducing extra instability under heavy braking or making longer braking distances, depending to which direction such brake swap shifts front-rear brake bias. In most cases for daily driving stock brakes are best choice. One already has them "for free", they are more then sufficient for most uses except long sessions on track, have much better wheel clearance allowing fitting most wheels including downsized ones, there are plenty of different pads for any taste available, and they are certainly MOT legal (for example here in LV legally most BBKs won't let pass MOT), and manufacturer engineers have spent lot of time to get brake bias fit best for this particular car. Not sure why you need to know piston size/master cylinder size and other data for BBK upgrade. One just should choose BBK designed for these car models by reputable vendor that should dial with keeping brake bias close to stock, then all you have to care for, caliper wheel clearance. For later on BBK manufacturer sites usually there are downloadable brake fitment templates, which one can print out, glue on carton, cut out and check with own wheels or ones that planning to buy, if they clear particular BBK. Or if vendor lists wheels of checked clearance, or if such info on particular kit can be found in forums. Who are "they"? There are several manufacturers/vendors making/selling BBKs for twins, often even several BBK models. There are plenty aftermarket parts shops that sell parts online. Yes, some BBKs are front-only and that's OK, if they are designed to work with stock rear brakes & retain brake bias (as fronts do most braking, and in this fashion one can reduce upgrade costs). Some upgrade both fronts and rears. There are many kits available for twins.

Strange, I've only heard good things about their lines, they are very popular in the new mini space and not heard of any failing. Hope you get it sorted.

Almost a year on from this post and I never got around to fixing it myself. But last month I was looking into the problem again and found that they sell these little clips on Japan Parts, and they aren't charging 15 quid each. Instead, I was able to get 2 of them for 17 quid, delivered. Overnight parts from Japan, yo! Although not overnight. I ordered on the 19th May and they turned up this morning. That's a little job to attend to in my time off this week

I'm waiting to order STI sideskirts for my BRZ again a US/JDM only part so its good to see which of those sites above people have used and had good experiences.