You fire at someone, they get scared, run for cover and hide. If you fire on full auto it's much higher penalty to resist. All is well there.
Lascarbine, S/2/-. Fired on semi auto, can force a full 45 degree killzone to panic and take cover. 2 shots fired. If it's a multilaser or some other badass weapon shooting more at the same time, it seems ok, but even a m36 lasgun only fires 3 rounds.
If it's 3 rounds from an autocannon then I can also argue that the armour tearing rounds would be enough to make you go into cover despite no real swarm of bullets, but all in all it seems suppressive fire should either be on every shot taken (simply because you're being fired at, it's scary) or only when the amount of bullets are overwhelming.
And yeah, technically you might have a comrade helping you out doubling it to 4 or 6 bullets, but the rules are no different for a stormtrooper with no comrade, or a guardsman with a dead one, than for the one who has a comrade.
I think it would've been better with some kind of cohesion system or similar like in deathwatch, in that you could make a (perhaps sweeping orders?) covering fire maneuver when a lot of the people in the group fires in the same area to actually produce that effect. As it is, it just seems like, I'd personally be more afraid of a guy firing 3 rounds AT me, than a guy firing 3 rounds in my general direction (and just MIGHT hit me or someone else randomly). But maybe thats just me