One thing about tacking in a big breeze is there is a natural equalibrium between tacking and coming in off the wire. It is much easier to tack from the wire at full speed than it is to come in off the wire, then tack.

In 15 knots of breeze you need to be moving full speed double trapped when you start the tack. The act of heading the boat up into the wind will reduce the heeling force allowing you to come in off the wire WITHOUT easing the mainsail. As your boat passes head to wind you (the helmsman) will probably be going across the centerline of the boat and finally easing the mainsail (just a foot or two). As you start to bear away a little beyond the new close-hauled course it will neccesitate your going out on the wire to counter balance the new heeling forces. Sheet the main back in and carve back up to a good close-hauled course on the new tack.

In my experience the key to doing this successfully is being smooth, turning the boat relatively quickly before it has time to slow down, never letting go of the tiller, getting across the boat really quickly, and starting and finishing the tack on the wire.