I've been playing a Dual-Wield Destroyer on VH using a high-damage (but slow) weapon in right-hand and a "Fastest" attack speed pistol in the left hand.
Basically what I found is, and sorry if this is a topic that's been beaten into the ground already on another thread, you will always attack with your ranged weapon at that weapon's designated speed if there are no targets in melee range. The reverse is true of melee targets - you will always attack with your melee weapon if there are targets in range in the direction you are facing.
I'm a big fan of Spectral Echo so I gave I put in a point as soon as I could with this build and made another realization:
With SE activated, your pistol attacks fire off arcs of damage like normal but they do damage relative to the DPS on your right-handed melee weapon. Everything happens so fast in this game, and I'm on XBOX so I don't think there's a way to record the damages for a real comparison, but I switched some melee weapons around playing with this and the damage on SE waves changed when my melee weapon changed (even though I wasn't attacking with that weapon).
Using this method, you can find that massive DPS 1-handed weapon and not really care what attack speed it is. As long as you have a fast pistol you can spam out damage so fast it's ridiculous. I just popped lvl 40 on VH (main story completed and down to lvl 25ish in SV). You slow down if you get anyone into melee range, but that's what Entropic is for. I'm not Hard Core but this character has not died yet. DW Epic Leone's Pistol and Epic Double Dmg! Axe and got through Aldrak in less than 5 mins.
Hope this isn't a bug that I've been exploiting rather than a game nuance. Anyway - the faithful may have realized this years ago but I didn't see it mentioned in my quick browse of the forum. Great game!