I have Boxster fronts .351.421 & 422 are the part numbers. They have 36 & 40mm pistons the same as the LCR ones. The standard Boxster disc is only 298mm in diameter which is why there is the unswept area on the 312mm discs.
I fitted these due to several reasons:
1 - I could increase my breaking performance but keep the discs/pads cheap. Plain 312mm discs can be got for £30 a disc
2 - The callipers only cost me £180 as they weren't a popular option for strapping onto this chassis (probably due to all the forum bashing they get!). I agree that by the time I'd had them refurbed and I'd bought all the bits I needed that the price was similar to that of the LCR setup.
Now I don't do many track days and neither do most people on this forum who want a brake upgrade. But the track days I have done I've never warped a disc and the only time I've suffered from brake fade was when I was running crappy fluid. At my last track day outing at Bedford Autodrome I was lapping as fast as 2 other S3 owners from this here forum. One of those owners being Stacey who as most of you know has a BT setup and massive Cayenne 6 pots. The other being Tam who has the LCR setup. Out of the 3 of us Stacey was the only one who warped a disc on track and for those of you who want to know the price of replacement discs on the Cayenne setup, I suggest asking Stacey once he's stopped crying
I have also driven Tam's car so I have had a comparison of both brake setups (before anyone has another dig).
If I knew then what I know now I'd probably have gone for the LCR setup but only because it would have worked out a similar price and looked slightly nicer without the unswept disc/pad area and nothing to do with performance.
I'm getting a bit tired of the bashing that seems to be going on at the moment, people shouting out about what's best and don't do this and that. It is true that simple physics state that the LCR setup is slightly better performance wise then the boxster setup due to the larger disc but I don't think it's that noticeable on the road! At the end of the day the carriers, piston size, braded lines are all the same and we all know that you can make any brake system a lot better by your choice of pad and fluid.
At the end of the day the Boxster upgrade is still a worth while upgrade. It seems to stop my 300bhp ton and a half car well enough for road and amateur track days. If you can get the Boxster setup at a real good price and it doesn't require new seals and a powdercoat (I could have just painted mine and bolted them on) then don't be afraid to go for it, and don't feel like it's a bad choice because others on here have slated it. It works.