Neil, I bought the 16" full moon discs and have had them on my car for about 6 weeks. I posted pics in the "Racing Discs, Are They Worth It?" thread.
I can't say I can discern any difference in mileage as the air temps around here have dropped about 15-20 degrees since August. This should have a larger, negative effect than the discs' positive one. As the air temps drop, my mileage follows. It's been this way with every car I've ever owned.
Anyway, mine are solid with no access holes to check tire pressures (there are places inside the cover (gaps in the 'teeth') for the valve stem but you have to remove them to check the pressure. However, I think this will take just a minute or so with a flat-head screwdriver.
Drilling through them should be possible as they are fairly thin metal ... but I would not advise it. I'm not sure the benefits outweigh the damage done to the not-exactly-cheap discs.
