I went with 20mm as didn't require to grind down oem studs. The OEM Si wheels have a small pocket where there is enough room for these to mount flush to hub with no need to grind. With 15 you may need to grind down depending on wheels.Looks good! I went a little more conservative and got 15mm all around. Installing them once I purchase my sport spec coilovers and get those in.
I'll do some before and after pics too.