I have now fitted the GB640s. I bought a 20mm tapered reamer to widen the holes. It is a tight squeeze but I am very pleased. I did ask Andyjr1515 for his advice and he advised using a crafty knife to scrape away the paint from around the hole before reaming, ream and then widen the inside of the whole with a round file or multitool. .It worked well except I did scratch the headstock with the multi-tool. So what were the advantages and disadvantages?
The light weight is great but the real win is the smoothness of the Gotohs. In addition the size of the 640s mean the original fixing holes are hidden. Although the posts are much wider than the original ones, the strings still travel in a straight line from the nut to the machine heads.
The downsides? It is a lot of work.
Would do it again? Yes but I would have bought the Gotoh GB528s. There would have been no reaming, just new fixing holes and there would have been some existing fixing holes to fill. Of course there are some Hipshot Ultralites that may fit exactly.