Just replace the rotors instead. Here in Blighty, getting the rotors turned
costs about £10 and new ones are about £20-£30, (pt cruiser, ford granada
and the jag) factor in the added downtime, needing another car or a friend
to take the things to an engineering shop and then fetch them back again and
the new ones can work out cheaper.
A problem I've had with vented discs before is that the cooling vanes down
the centre of the rotors can crack and cause different expansion rates along
the disc, which will run true when tested cold but then warp when hot, and
cracks are regarded as 'a bad thing'
Also, when trying to stop nearly two tons of car from three figure speeds, I
want the things to work, more metal = more cooling = less fade