Typically, there won't be much change in speed if you use appropriately weighted arrows for a given draw weight (i.e. 5grains of arrow weight per pound of draw weight).

First off, Hoyt bows are tested according to ATA methods, which is exactly 70lb draw weight, exactly 350grn arrow, and exactly 30" draw length. For shorter draw bows, the max draw length is used (i.e. youth or womens bows), and for lighter draw bows, peak weight is used, with 5grain per pound arrows.

Relatively speaking, to calculate the change in KE when changing draw weights, you can get pretty close just by factoring the known KE number. For example, if the Carbon Element ATA/IBO's at 330fps, that means it will throw a 350grn arrow at 330fps when set at 70lbs.

That's a KE of 84.62ft.lbs. If we simply multiply that by 8/7's, we estimate that the NEW KE for 80lbs will be 96.72ft.lbs (84.62 * 80 / 70 = 96.72). However, when set at 80lbs, the bow will now require a 400grn arrow. If I back calculate the arrow speed from the new KE, I get 330fps (sqrt(96.72*450380/400) = 330).

So I wouldn't necessarily expect any increase in speed, but a fairly significant increase in KE.

Now, that said, bows DO get more efficient as you approach peak weight (or LESS efficient as you turn them down from peak weight). So I do expect that the bow will get slightly faster if you have it on 80lbs vs. 70lbs, but not much, maybe 5% more KE, meaning 2.4% more speed.

On the flip side, using the same arrow weight, say 400grn (minimum arrow weight for 80lb draw), you'll see a huge drag in speed at 70lbs. As mentioned, the bow should push 400grns to 330fps at 80lbs, but at 70lbs, it will only move a 400grn arrow to 308fps (based on 84.62ft.lbs. KE).