All of the powder is burned within 6-8" regardless of what powder you use. A faster powder might completely burn within 6", a slower powder might need 8". Whatever powder you get the best speeds with from a 26" barrel, will still give you the best speeds from a 16" barrel. A faster burning powder doesn't help give you more speed from short rifle barrels. Pistol length barrels under 8", yes.
The 16" barrel will be slower, but that is because the bullet spends less time in the barrel and the expanding gas from the burning powder has less time to work on the bullet.
Generally heavier bullets need slower powder for best performance. In 308 that means bullets heavier than 180 gr. I mostly shoot 150-180 bullets in my 308 but have experimented with 125/130 and as heavy as 200. Varget is my go-to choice, but there are a LOT of good options for 308 with common bullet weights.
What little experimenting I've done with 200 gr bullets tells me Varget isn't the best choice. I need something slower.