The spindle mod got me about 100 RPM, not near enough.
The stock high speed pulley is a 1 to 1 ratio and the pulley is about 2.35" in diameter. To go from 2500 RPM to 4000 RPM I would need to make the pulley 1.6 times larger or about 3.76". There is just not room for that with the stock motor mount in the way.
This is the pulley size that I think might fit without molding the mount.
Its 2.73" in diameter. That should give about 3000 RPM.
I had a 10mm reamer so a simple drill and ream gave us a nice bore that fit well.