What is an unmodified hit roll?
"Dakka! Dakka! Dakka!" will still apply to the 2 "natural" 6s.

The wording can be a little confusing, but Warhammer uses "unmodified hit rolls of 6" and "hit rolls of 6 before modifiers are applied" interchangeably. 

Essentially it only cares about the number showing on the dice and doesn't pay attention to modifiers being applied.

The place this is most clearly set out is in the Designers Commentary