because porshe brakes are not brembo brakes.
the porshe calipers are slimmer than brembos.
Which Porsche calipers?? And yes they ARE Brembo brakes, well all late Porka calipers are, be more precise.
LCR Brembo's are slightly custom GT Junior calipers with VAG designing the carrier to suit the VAG hub and disk arrangement.
There are literally 100's of different variations and applications most of which can, with the correct mounting brackets and choice of disks/bells, be made to fit.
My Porsche calipers and other ones I have had direct experience with have been wider than the GT/LCR brembos so I dont know where your comment comes from.
There is no definitive answer to this as there are so many different combinations to choose from. Disk/hub offset, wheel offset, wheel width etc.
As I've already said, you could be looking at 15mm spacers to clear the back of the spokes. That's what I needed with Porsche GT3 calipers and 10mm with GT's and R32 brakes.
If your budget allows then remember that LM are split rims so you could order a set with exactly the right dimensions for your application but dont expect change out of £2k.