Couple of points here.
First- I strongly advise AGAINST just arbitrary upsizing the wire one size. I hear this a lot - BUT-
the rig on a sailboat is a system, from chain plates to mast head tangs including turnbuckles and clevis pins. Often an increase in wire size means the ends are larger(stalocs, swaged eyes, whatever) requiring larger clevises. So you bore out the hole in the tang or chain plate to fit? And maybe cut the strength of the fitting to LESS than where you started.
So before you just jump a wire size, consider the whole system, and be sure it's really needed. Modern 1x19 304 or 316 stainless is extremely tough stuff. Do the math first.
Secondly, I'm not really up on the CD 27, but right across our home slip from us sat Ed Campbell's CD 25. Ed is the author of Cruising Texas, which used to be the only cruising guide for the Texas coast. Ed has sailed that boat, in total stock form all over the Gulf of Mexico, including several crossings straight across the gulf. With no difficulties at all with the rig.
So, as I said- do some research before you go changing things.
After that, I agree with Adrift on the other stuff he mentioned. But again-do some serious thinking about how you'll REALLY sail the boat. Are you REALLY gonna cross oceans? Or are you really just gonna do coastal sailing? Personally, I'd get the boat ready and sail her a few hundred miles before I did any "improving"
of course, if you already know you are a second James with Atom, then pull the stops and go for it