Crit is the greatest factor to wildfire damage.

I'm sitting at 578 skill speed (2.41gcd) and ideally, I'm not overshooting my refresh on hot shot/leadshot or getting oGCDs inbetween the period where I don't have hot shot for that inbetween GCD. I;d imagine having higher SS would cause you to overshoot the refresh by one tick on lead shot. Other than that, I haven't seen any official statements on patch notes saying that skill speed affects turret damage (only spell speed in regards to summoner pet).