Mick, Does you Endurance have a full or shoal keel, this would be the biggest difference in how she heads to wind, but also I’m not sure what angle of attack you are talking about to apparent wind. Also, how she is loaded is going to effect her performance as well. But when in doubt sheet out the head sails and leave the main sheeted tight. this will use the headsails to make the main more efficient. Also depending on your angle, you may be cutting off the air to your mizzen, so falling off to say 30+ degrees may be much better headway for your boat. Swell, usually doesn’t make that much difference in actual headway, unless your sail shape is off.
Hope this helps… Mark