Sadly, air resistance limits how fast you can go, and even if you could eliminate that, the speed of light would still stop you.
While storing over a period of time, Wayne has a lower rate of healing. health / time * time = health. Actually, I'll change it and call it "healing" instead so that's clearer.
So, if you can only store 75% of your speed, then when your speed is zero, you won't be getting anything. Luckily for Sazed, it is speed*time, not velocity*time, so every time he shifted, the metalmind would fill more. Only being able to store 75% also means that you definitely benefit from going faster while storing.