I have a feeling that this may well be more
rig tune than anything else. If you have weather helm when under headsail alone the it sounds to me like you entire
rig is too slack. How is the forestay tension ? If it's slack then tighten up on your backstay and see how you go. One other thought is headsail block position. You might need to bring the lead further aft in order to flatten the headsail. Once you have the weather helm sorted out for your headsail then move onto the main.
Your now tightened forestay will help to flatten the main and so you may well find that all is well. If it improved but is still more WH than you'd like , and having checked your outhaul and halyard tension, then where is your mainsail sheet block ? In over 15 knots the car should be right down to leeward so that you depower the main but can still keep the boom low. Judicious use of your vang will also assist in keeping boom under control as well.