So perhaps the reason we cannot find a general rule for it, is that it is keyword specific.
Page 30 in the RRG looks pretty definitive:
"If a unit performs an attack using multiple weapons that have the impact x keyword and those weapons contribute dice to the same attack pool, the x values of each impact x keyword are cumulative."
I have 5 weapons that add 5 black dice to my attack pool.
If the argument is that the unit only has 1 Impact Grenade Weapon, then I would go back to page 44 under Upgrade Cards:
"Some upgrade cards feature the hard point (?) or grenade (?) icons; these are weapon upgrades. These upgrades do not add any minis to a unit; instead, every mini in that unit may use that weapon when attacking the enemy."
EDIT: Ahhhhh - okay, I think I see where you guys are coming from. "Every mini in that unit may use
that
weapon when attacking."
It's still only 1 weapon, so you only add the Impact 1
once
instead of for each time the dice are added. This is different than Speeder Bikes because each mini is using a separate weapon with Impact 1.
Consider me learned and awaiting an FAQ
Edited by RhymerNation