|Without seeing your entire setup, I can only hit a few high spots. I'd be inclined to secure the tack right at the stemhead and put the pennant/tether at the head to improve the end plate effect.
Everyone at my club sets their main shrouds very tight ("If you can open your cupboard doors easily, tighten the shrouds some more.") and set the forward lowers just tight enough to prevent inverted bend in a blow, no more; the aft lowers should be snug enough not to wave in the breeze and little more.
Finally, have a look at your spreader inboard ends. Do they hold the spreaders firmly fore-and-aft or can you wiggle the spreaders back and forth? If they wiggle easily, they will move fore or aft under load, which loosens the weather shroud, thereby negating all your work in tightening. This will contribute to a failure to point and probably, to different tacking angles on the wind.
C&C 27 Mk IV