I was told it is a 2 week wait from BBS to get wheels 'machined' by THEM for the change in ET.
Anyway, that's for mid Sept before the wheels are delivered. I'll certainly consider what you have alerted me to. 
My fear is that a mounting face has a steel seat (usually) with a set depth below it to allow the use of standard length wheel bolts...if you machine off 5mm from the face, does the bolt extend through the hub far enough to impact on the ABS ring?
There are also issues of bolt torque as the given torque is based on a set clamping length...if you remove 5mm, you shorten this by 5mm and spread the torque over a 5mm shorter distance...does this impact on the bolts elastic limit?
Probably not...but it just seems odd.
I know for a fact that an ET35 CH is a different forging/casting to a ET50 CH...so I find it hard to believe it's all in the machining.
Bear in mind too that they'll be taper seat wheels, not radius seat, so you will ned the matching bolts from BBS, and your locking wheel bolts will not fit....and you'll need to keep 5 of the standard bolts in the boot in case you ever need your spare!
You maybe knew all this...just pointing it out in case.