It is importand that the boat stays calm, to keep the flow attached to the sails. No slapping in the waves. Every movement of the sails and the mast hurts. Our 18HT is very sensitive to movements due it's light weight and the tall mast.

I found for my self, that when there is a small ground swell running, the boat stays much calmer when we are sailing on one hull. We do everything to stay on one hull when the boat shakes. We sometimes even trapeze to leeward to avoid the nerve breaking slaping of the windward hull.

I also read in some A-Cat go fast manual, that the drag is increased by 10% when sailing on two hulls. I don't know if thats true or not.

When there are no waves and maybe some small puffs, I agree that it is faster having both on the windward side. It's not good when the crew has to climb up to the other upper side, to keep the boat down.

Of course, the stern MUST be allways out of the water.
Keep your eyes on the mainsail lech tell tales. They should fly for around halve the time. Over sheeting is very slow.