Right, I've just noticed this thread, and seeing as I have first hand experience, I thought i'd add my views.
I had standard j-spec brakes, disks and pads. I've just been down to do some measurements, although my setup is different and doesn't require any spacers, as I used UK Spec disks (although they're grooved DBA's), not the evo disks. I used Evo pads (red stuff) and Evo8 calipers. See attached pics for rough measurements (excuse the brake dust )
The weight difference in the calipers was quite large. Using a kitchen scale (when fitting) I measured the J-spec calipers at 2.8kg's, while the Brembo's never even broke the 1kg mark (see pic). I didn't weight the disks.
The measurements are:
5.5cm from disk surface to outside of caliper
12.5cm from centre of hub to widest part of caliper (roughly the centre)
Excuse the pic quality - mobile phone
All I can say is the difference is truly amazing from j-spec to Brembo. I can't comment on the UK brakes as I've never tried them.
Cheers
Mark