I wouldn't even try to compare with polars as these are all taken in a new boat in perfect condition in a flat sea with steady winds &a pro skipper. also if you want better performance ditch the in mast system & get a new main either fully battened or (better) top 2 full bottom 2 leech only. this gives better shape control. 3 knots overall? you should get 7 to 8 on a fetch in 16 to 18 knots windspeed – under 12 knots this kind of boat will not fly – also sea condition has huge effect! sorry, but if you want to make port in a hurry you'll have to motorsail – we all do! also as mentioned a fixed prop will take a knot off, also a dirty bottom. if you need more reliable speed get a J and keep the weight down. the
average cruising yacht (dufour or no) is built for comfort these days – I've had both and if down to 3 knots always motor – sad but true….
best of luck
dave (D39 integral)