I can confirm that it increases the respawn rate in respawns with Varying Monster Spawn system.
Now this happens due in those respawns none of the mobs have a position set which doesn't allow to chose individually which position should have increased spawn rate, which turn all the position where that could spawn into a boosted one, but this only happen in these specific cases where isn't possible to increase the spawn rate of an specific position, so for most of the hunting spawns this isn't possible.
I will try to get a CM answer for this later, but mine was also personal experience through hunting at Oskayaat Weretigers which had recently White Weretiger boosted.
Update: beside CM Liamas couldn't confirm how it works or impacts he was able to confirm us this:
I have now received feedback and without being able to go into more detail, it actually looks like the boosted spawn rate influences the general rate.
Which means that yes, when a boosted appear in a varying respawn it will boost the overall spawn rate:
Here the link to the actual answer:
Answer From CM Liamas