2 bre +1vickers have 21.4 DPS at max range
3 Vickers-K have 23 DPS at max range
2 Commandos sten have 24 so the difference is negligible
Tech does not say much but the unit cost is significant. And commandos are doctrinal not stock.
In addition Heavy sapper also get armor and repair speed with it.
As mentioned before the upgrade is badly designed not bad.
The argument is why should anyone pick special weapons doctrine. Triple Vickers sappers is clearly not one.
In case of doctrinal triple vk sappers they cost more than double the muni for less DPS even at their specialist range.
Movement nerf and unable to fire on move vs unit that can fire brens on the move.
Vk might be worth it on tommies if they didn't need two requirements for it. (Meanwhile svt cons, m1919s, StGIR look and laugh)