I don't think you should overdrill holes. You should use a large doubler that is thick enough not to bend. A piece of 3/16" or thicker aluminum 3 or more inches in diameter as a washer is a good start. Overdrilling and filling doesn't get rid of the stress concentration of a motor mounting hole. But a large thick washer does. Or the C-channel I mentioned. (which I wholeheartedly recommend) It will also reduce the compressive stress on the coosa and make the whole thing much more durable.
You should use unwaxed resin all the way thru unless it is your last coat of gel coat. If you have more than 16 hours between coats, put down a layer of peel ply. Then you just peel it off next time you work on the boat. No sanding needed.
Sandblast the transom before you cut into it rather than sanding. But first acetone wash it. Then grind in your taper with a disk sander, then wash again with acetone. Then cut into it. Sandblast the interior of the transom after the wood is gone. It is fast and it gets into every corner. Then vacuum and water wash and vacuum and acetone wash.
Personally, I don't like epoxy. But I can't say it won't stick. Although you might check to see if the vinylester will stick to it, else you will need to make sure very little (just the saw kerf on the outer skin) comes in contact with the outer layer of vinylester and glass. A test batch will tell you this. Some vinylesters will stick to epoxy well.
Also, if you can make mega C-clamps like flyingfrizzle to clamp your transom, that's better than drilling lag bolt holes, obviously.
Those are my suggestions.
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