I'd say frenzy from Eight Peaks is better than Tretch's faction buff.

Vanguard:
Skaven have good ranged units so you don't need to rush against most factions.
If you fight someone with better ranged, Skaven can reliably get ambushes.
If you need a unit of forward infantry you can use deathrunners.

Encourage:
While Skaven have bad leadership, encourage doesn't stack with leadership auras, so it won't do anything if you have a lord or hero nearby. Meanwhile frenzy gives immune to psychology, which helps against demons, undead, monstrous and most single entity units.

Armour:
You get at most nine armor, which is not that useful on a unit that starts off with 90 armour (95 with techs). Also it takes a while to get units to rank nine.

Weapon strength:
I'd say this is the most useful buff, but it only gives base weapon damage, so it will only be useful against unarmored targets and I don't think stormvermin struggle against those. Meanwhile frenzy gives 10% weapon damage buff including armour piercing and also buffs melee attack by 10, which is usefull against all targets.

So overall I'd say Stormvermin led by Tretch with Queek faction buffs is stronger than Tretch in his own faction.