I tend to think closer to Thaago and Retry. If your total flux usage exceeds your flux dissipation significantly, you don't get the full DPS since the weapon has to not fire for long periods of time, despite being in arc and range. A heavy blaster has 500 DPS for 12 OP (41.6 DPS/OP). However, slap that on a Wolf with only 250 flux dissipation, and ignoring everything else, the heavy blaster at best can only fire 34% of the time the ship is deployed. Dropping that 41.6 DPS/OP down to 14.1 DPS/OP for sustained engagements, or less depending on other flux usage constraints.
Which isn't to say it hit-and-fade tactics can't be effective, but in fleet fights, there are often situations where you don't need to fade, since another ship is the focus or you're using EMP effects to limit return fire. I sometimes think of the total flux available to a ship in an end game fleet fight (i.e. capital grind) equal to its vent rate times it operational time (plus a factor for how often I manually vent). Since it is very easy to simply be continuously engaged in such fights. So at what point in the game, or what your expected opposition is, can influence this evaluation.
For weapon comparisons, I generally think in terms of an equivalent OP cost. Namely the weapon's OP cost plus flux vents to make the weapon flux neutral. Plus any hull mods that need to be added split between all weapons on the ship to make said weapon effective (applies more to missiles, but some game mods change this). So in that view, a Heavy Blaster effectively has an OP cost of 12 + 72 = 84, for about 6 DPS/OP.
This view does has the limitation that as you eventually hit the point when the ship's vents are maxed out and you can't really get any more flux for OP, but I think it gives a better feel for the tradeoffs. Both OP cost and flux efficiency matter. We're also completely ignoring effectiveness against armor in these numbers. Against ships with cruiser tier armor (1000 armor), a heavy needler is going to be doing 33% of its listed DPS to hull after the armor is gone (because of the minimum 5% armor reduction). Mark IX Autocannon is going to be doing 66% of its listed damage. Heavy autocannon will be doing 50%.
So for this particular comparison, I'd rate the Mark IX Autocannon as 348/(18+40)=6 DPS/OP, while the Heavy Needler is 250/(15+20) =7.14 DPS/OP, and the Heavy Autocannon is 214/(10+21.4) = 6.8 DPS/OP. Again ignoring things like range, accuracy, and armor penetration.