[gpm]

For normal moderately inclined orbits the boost is still greatest at the equator, just always less than for non inclined orbits (until you start launching retrograde).

The soviet launch site was indeed chosen for other reasons than maximizing payload... in particular I think I recall that it was largely motivated by avoiding launching over China, and being in an empty enough part of the world that it's unlikely to kill to many people (despite launching over land).