From what I understand, I'm not sure if the sync directly affect the secondary stats themselves, but it definitely does lower their cap down, essentially the ilvl sync just caps your main stats to that on an i150 piece, this doesn't necessarily apply to secondaries though, as using i150 pieces will result in lower-than-average secondary stats.

Reason being, theyre too low at that point to effectively hit the cap. General rule of thumb for me has always been that the ilevel required to cap out all secondaries on a specific gearslot was the ilevel sync + 30, in otherwords, a mix of i180~i210 gear, be it PvE or PvP, is best.

Typically you want to focus on stacking as much crit and det as possible, though some may opt for skill/spell speed as well.