Jami, you recently changed to upper and lower sheeting. I wonder if now you have enough pull on the upper part of the sail to oversheet it? As I said here, I find that at full sail, I need less tension on the upper sheet. The best way to check this is with leech telltales - about half a metre of ribbon at the ends of the yard and upper battens. They should all stream aft, and if they are collapsing behind the sail, then the upper sheet should be eased. Yes, more balance = less helm load, but there should be no effect on the lift/drag ratio.
But you're talking about 16 - 20 knots of wind, and if your sail is just too big for the boat, and you're not reefing very much of it, could it just be the case that you're over-pressing the boat, and as is usually the case, when you take another reef, you go faster?
Having said that, I wonder whether you're also simply experiencing the same feeling as I get with Weaverbird's sail, that the lower angle of the yard (subjectively, if not objectively) just makes everything seem easier? Anyway, as Arne says, if it works to take the top panel off, then go for it - after, of course, testing in as many conditions and over as long a period as you can. Do nothing too hastily.