45 Dwarven Lapidaries = 45 gems/sec base

2 Brewmasters give +250% efficiency, so total efficiency is 350% (3.5×)

45 × 3.5 = 157.5 gems/sec

So 45 Lapidaries cut ~158 gems per second with 2 Brewmasters.

https://prnt.sc/kspq7Rwc92iY

But it's only 2.5× 

I noticed it too, it seem to be counting the first 100% as the normal output, so the first upgrade puts you to 125%, meaning that you only get a 25% increase for the first upgrade, and then, you got the 125% in the next upgrades as it should be.