George:
I would not make the holes larger as this would seem to just give more access for water into the area. As most know, I have a different perspective on the chainplates, and now after having my 1994 chainplates removed, my perspective is not flawed. Owning IP35-165 since 2001, and saiing her Maine to Bahamas 3 times, I have to tell you that I have never pulled the chain plate tops to repack and recaulk them! Yup, that is true. I simply keep them sealed from the top and with my varnish.
Now after 20 years, the chain plates were removed by the factory and yes, I had a small leak on the forward starboard side, but guess what, no real issue with rusting or the welds. I professionally think, and radically so, that this water intrusion and or water trapping has very little to do with failure. I think it is just age related and the variations of steel and weld quality and other topics.
So, I would NOT make your teak openings any larger. I would NOT bust my tail in repacking, I would keep them sealed from above, KISS, and after 15-20 years, I would have them replaced for $9-$10K by the guys who installed them, the IPY factory!
Hayden