Increasing your drawlenght means your bow will deliver more force. Some say that is 2,5 to 3 lbs per inch more. That's pretty close for bows around the 50 lbs.
I always calculate 5% per inch more. If a bow is 50 lbs @28", it will deliver 5% more at 29 and that is 2,5 lbs indeed. So at 29 inch that is 52,5 lbs. At 40 lbs it will be 2 lbs more and at 60 it will be 3 lbs more.
What does this mean regarding to arrowspeed? Well, increased drawlenght means more lbs, making your arrow relatively lighter.
A calcuation: Suppose you have a bow of 50lbs@28". Your arrows weight 500 grains. Then you will shooth as arrow weight: 500/50 = 10 gpp.
Suppose you draw 30". Then the bow will deliver 10% more lbs: 1.1x50 = 55lbs. Your arrowweight wil go down: 500/55 = 9,1 GPP
A difference of 1 GPP will give you 10 fps more speed (as an average, depending on your set-up)
However, you won't shooth the same arrows when drawing 5 lbs more :)You will have to go higher in spine value and that will make your arrow heavier, loosing the advantage of an lighter arrow.
Or you can lower your front load (broadhead, insert or fieldpoint) with 25 grains to adjust the 5 lbs more, making the arrow even more lighter and giving more speed. But this will affect your FOC (making it lighter)...