Is a tricky business to "generalize" (is that word right in english!? jajaj)
It depend on what impact do you have, what speed do you want and what projectile you will shoot ... and finally are we talking on stock impact or modded impact? (will not go on modded impacts). Stock ones:
Impact X, valveseat (VS) 5.5mm, ValveTube (VT aka plenum) 20cc
Mk2 Early Release, VS 5.5mm, VT 25cc
MK2 Latest release, VS 6.35mm, VT25cc
As you can see all this changes affect the speed, precision and harmonics...so and FOR example, to achieve 900fps you will need different configurations for those 3. The plenum might be needed or not, adding an external plenum add more regulated air volume, so in theory even achieving 900fps on the pellet/projectile you choose and without the plenum, the plenum addition will help to lower your regulator pressure, ergo your tension on the spring, ergo the hammer strike (less force to open the valve), that can lead to less vibration and better accuracy ... but like I said in THEORY ... your Impact might achieve the "optimum" without a plenum, and adding one will improve nothing.
At the end, precision is all that matter is the ultimate goal (at least for me)
BTW: Adding an external plenum is not plug and play, mods need to be made...of course you can add one and do nothing but actually will not help as it supposed to)
First you get the gun, then you get the speed, and then you got the Accuracy!
(scarface mode on!)