Since your Center of Effort will move up and slightly to the rear, you should experience a little more heeling moment, a greater tendency to round up in gusts, and a very slight increase in weather helm.
Inasmuch as windward ability comes mostly from the luff edge of the sail, wouldn't you be better off to keep a long luff edge to the sail, and instead of a deck sweeper, have a sail with a higher cut clew?
If, when using the "pennant sail", you elect to not use the pennant, but attach the tack directly to the deck, you won't need re-attach the pennant to the head of the sail, the halyard will do just fine. Will you be faster with a lower jib? I don't think so, wind is usually stronger higher up, so a higher sail should catch more wind and therefore be faster.