There is quite a bit of difference between STF Normale and Elite players.
Everyone who is confident in his ship's abilities and build won't bother with the normals.
I always run Elites straight away even with standard white gear if I have to.
So you really get the bottom of the barrel players who don't really know how to sqeeze some performance out of their ships in the normals.
That costs you a lot of synergy.
Borg are pretty much resistant against everything but not specifically more resistant against plasma than tetryon.
You will cut through shields a bit faster with tetryons and hull typically is dying soon after from the heavy hitter torps.
Plasma is a good pressure weapon to force an enemy to blow his hull heals before his shields go down.
But NPCs in general and Elite STF Borg in particular have so much hitpoints the effect might only be noticable on the probes.
And it's not just the spheres that are faster. A lot of NPC ships have gained a EPtEngines spower. But shperes are cruisers and don't have the turnrate to use it effectively so they always appear to zoom away from the fight (and sometimes get stuck in asteroids or other objects).