machineing wheels for more ET?

[Erik R Addy <erik@aero.und.edu>]

Does anybody out there have good knowlage about machineing the
mounting face of alloy wheels to gain more offset?  I am considering
purchasing some OZ Mito one piece wheels with ET35.  I think I will need
to machine about 5mm, possibly up to 8 to achieve a ET40-43 to fit this
wheel with a 225/50-16 R1 or 032R under my custom flared rear fenders.

My questions:

How much can you safely machine?  I assume this can vary from wheel to
wheel, but is 5mm reasonable? 8mm?

Does a wheel that requires a adapter ring to be hubcentric make this any
more difficult?

I assume the wheels need to be unmounted (no tires) to do this, right?

How much does this generally cost?

Thanks,
Erik
erik@aero.und.edu