Unless the ROF is capped between 150-180, that would basically just result in an inevitable buff to Last Word (not that I'm against it since I love it). Fan Fire makes it full auto (TLW is a 180 in the database, so this increases it to 225), increases hip fire precision damage and increases reload/ accuracy on hit. All but the full auto are a result of the hip fire rework it got years ago, which came with a massive buff to it's version of hip fire grip. Effectively granting it Enlightened Action and something similar to the Nadir trait. I wouldn't expect a legendary version of it to have all those bonuses, but it's what other perks we could expect on a legendary version of the weapon.