The issue is that the S15 6-speed takes its speedo reading from the ABS via a sensor on the diff. No speedo drive gear. Some of the S15 boxes have no provision for a drive gear at all. So, there is a key issue to overcome: hence why S15 boxes aren’t in extensive S30 use.
It’s a massive gearbox, weighs about 30odd kg more..
So what, other wallies put in R200s and steel calipers which weigh more as well !