I have been racing Daddio (RC-27) in PHRF now for a few years. We started with the conversion at the US Sailing site, which resulted in a rating of +6 I think. Each chance I get I drop the rating to something more reasonable. It is now at -90 and I can still kick the crap out of all of them if the wind blows more than 12 kts. The -90 's sweet spot is around 5-12 kts in smooth water. Offshore in waves and light wind conditions make the -90 nearly impossible to sail to.
The important thing is to not deprive any of the monhull sailors of their trophies. That will definitely turn them off. We sail in our own class sometimes and forego any trophy just to have a chance to show off. So far it has resulted in one Hobie 33 team at our club buying an RC-27 to race next year.
If we all are very courteous and don't whine, we can dispell the rumors most keel boat sailors beleive are true about multihulls and their skippers. Don't wear your wet swim suit or shoes in the club house, buy stuff from the bar and don't sneak off to your car to grab beer out of your trunk. Compliment the monohull sailors and don't always talk about how much faster your boat is than theirs.
All of these things will not only get you invited back, but will have them lining up to sail with you at the next race.
Pictures of Daddio in PHRF