Yes it was me, there are a few misconceptions though, the R53 calliper has exactly the same piston bore as the F and TF, the R56 calliper has the bigger bore but you need the smaller R53 calliper for this conversion. You may ask why bother swapping one 48mm bore calliper for another, its for two reasons, the first and most important is that the TF calliper was designed for 240mm discs, a 280mm disc doesn't fit into it properly let alone the 294mm disc of a Mini Cooper S. The R53 calliper was designed to fit a 276mm disc, a 294mm disc slots in perfectly and the edges of the pad don't overhang the disc. The second reason is that the brake pad is larger and will dissipate the heat quicker. The AP four pots aren't as effective as this setup either simply because of piston area, they are just 31.5mm in diameter. So pi x r2 is 15.75 x15.75 x pi = 779.3 sq mm but with four of them it totals 3117 sq mm. The R53 calliper is a 48mm bore so 24 x 24 x pi x2 = 3616 an increase of 14% but with a slightly smaller disc 294 versus 304 (3%) so the mechanical advantage is about 10%
You may have guessed by now that the brake disc that you need is a Mini R53, however there are two versions and you need the four hole not (obviously) the five hole one. Move the bolt holes 2mm inwards and some alloy spigot rings and you are almost there. I used some 10mm EN3b flat bar for the bracket and bought some stainless steel spacers from Ebay. I did have to grind a little from one hub mount due to it being cast on the generous side. Other than that it was a very simple job, no lathe or milling work and can probably be done for as little as £150, I paid £30 for a pair of callipers, then a rebuild kit, brake discs, steel bar, spacers and pads. I was helped out by selling my AP callipers for £200
For the rear I used MX5 276mm discs (bought with the left over money from the 4 pot AP's) again some AN3b bar, no spacers but a couple of washers. I had to open the centre bore by about one mm, easily done with a 60mm flap wheel in a fast spinning electric drill The standard TF rear callipers were actually designed for 260mm discs so a 16mm increase is nothing very much to consider. I actually bought new SAIC rear callipers but it failed the MOT because one was hardly working, I may experiment with MX5 NC callipers which may be a perfect alternative. I did read that the handbrake cable interferes with the subframe with large rear disc conversions but mine only gently touches when the suspension is at full droop. Although absolute braking performance is limited to the tyre grip nothing installs confidence as a rock solid pedal which is also helped by the servo brace and braided hoses which incidentally fit straight onto the R53 callipers.
If you are ever looking for something like these disc then Ebay is your friend, in this case I searched for "22mm vented brake disc" and the search came up with all the 22mm thick brake discs, Febi (the brand and Blueprint) is good because they normally list every dimension of their spare parts, so you can very quickly have a shortlist of what might fit, you can obviously do the same with diameter, centre bore etc
EDIT
I forgot to say that the only thing to do with the R53 disc apart from the hole moving is to get some spigot rings to convert the TF to Mini size disc bore.