To answer your question, the 20" barrel is really optimum for most commercial cartridges. They're designed around it. 24" & 26" barrels do give a slight edge in velocity, but not a huge edge. It can be argued that longer barrels stabilize bullets better, as well, due to the longer engagement between the bullet and the lands. Shorter barrels are more rigid, so they do have less barrel "whip", resulting in an accuracy advantage. The problem is that advantage is negated by the decreasing consistency in powder burn in shorter barrels.
This is going to get the flames going, but if accuracy is you utmost concern, go with a 20" barrel. If accuracy is a major concern but not the only one, don't go below 16". SPRs are often 18", a very nice compromise. Shorter barrels can be very accurate, Wes has some that shoot itty-bitty groups I here.
And I do know something about Tac-Ops. I have an Alpha 51. They recommend 24" unless you're an urban sniper.
Edited for spelling.