Eric,
I saw your post and hope we can get you going better upwind.
Judging from your discription of the wind speeds you should have been in great shape.
10-15 is a sweet spot for the ARC21 and 20 should not pose a big problem provided the boat is tuned correctly.
First a couple of questions:
What was the total crew weight?
Did you feel over powered or just no zip?
Double or single trapeze?
The ARC21 sail plan has gone through four designs. The very first was a standard 19X square top mainsail and a new jib. With this first design prebend was not used and the mast was rigged pretty much the same as the older 19X & XL series.
We felt that we definately needed a bit more horsepower so the mainsail was altered with more luff round. This helped a lot and we then changed the mast rigging to use pre-bend. The first pre-bend used was 2.5" measured at spreader root.
The next mainsail design was to add more luff round and to increase pre-bend to 3.25". This was the original sail plan that was sold to your boats previous owner.
Your first step is to determine your present sails' luff round. I am quite certain that the sail was modified at least once after delivery and this is quite possibly the root of your problem.
The mast pre-bend is critical to flexible performace and pre-bend will effect sail design. They have to work in harmony.
The last sail design was mainly a change to a newer material and a minor change in the upper quarter ( a revised luff curve above the spinnaker tang.)
Battens have a huge effect on sail power also and if you check length and compressions of each batten I can check those against standard design specifications.
The jib design was only altered twice , mainly batten placement and clew plate position.
Your jib was the second version and is essentially the same as the current design.
Hope this helps some but without being there and seeing conditions, weight and sails it is extremely difficult to be sure what to try first.
I would concentrate on sail shape & pre-bend settings.
I sailed the 21 for three years and last summer and in the wind speeds you indicate the 21 will sail extremely well. Certainly no problems with 18 footers.
BTW the traveler should be used extensively in those wind speeds. Just letting the traveler out and leaving it out is not a fast technique. My son and I sail at about 380# and occasionally with my grand daughter at 60#. We do notice a differance with the extra 60# ( my son does not trapeze as much). Plus she helps with spinnaker
Good luck and give me a call if you need more info.