I was concerned about this too, (before i just bought a new standard crank lol)

Cosworth themselves told me that they will supply bearings
to .5, which is where their tolerances finish,
and Still told me on a couple of occasions that .5 crank would
be ok at 500 bhp.

So i'd guess that going to .75 would probably be ok.
but to be honest the time and hassle of getting the 1mm bearings you are talking about you can just buy a crank thats already done.
I got offered a few on here before i got mine.