The keel isn't the major thing as there are lots of standard keel r930's running square top mains.
Too big a square top creates drag which negates the extra speed produced.
The good thing is that R930's have been doing square top mains for a while now and the sail makers arn't experimenting too much any more.
Get quotes from the major lofts and compare pricing and laminates. talk to the owners and see what they say.
airship and testarossa have pretty nice evolution square tops.
norths make a really nice square top too currently on no worries ( i think), pepe and a few others.
I have a pretty nice doyles square top on rafferty rules
I'd be more worried about the laminates they're using. The biggest difference in the last few years is that the laminates have gotten better ( apparently) with the new stratis and norths 3di laminates getting much longer life.
If you're pinging several grand on a laminate mainsail you want it to last several years of hard racing and the old laminates are not that durable.
I've seen fairly new ish laminate sails from various lofts fall apart sooner than I would have liked.