Ever since zekrom published his reverse engineering of the detailed damage formulaviewtopic.php?f=5&t=21317&p=199180&hilit=detailed+damage#p199180
I have off and on been experimenting with the impacts of weapon speed enhancements.
It appears that attack speed affixes, whether initial enchantments, player enchantments, or embedded chaos gems, DO NOT change the calculation of DPS. Only the initial speed class of the weapon seems to affect that. (In contrast, any added damage enhancing gems ARE multiplied by the weapon's speed factor =see zekrom's article= to arrive at the DPS increment.)
It appears that speed enchantments and chaos gems only increase the actual repeat speed on the act of firing. This can still be quite significant in heavy combat (especially if you are facing a big mob and relying on life leech to keep you alive through the skirmish.)
My most recent experiments, however, indicate that there is an absolute limit on attack speed, regardless of how much speed enhancement you cram on. The limit seems to be an Attack Speed value of 0.5.
This is the fastest value that is initially available on any weapon type in the game, specifically on Ribauldequins & Organ Guns.
Even when I add as much as 25% total speed improvement to a Ribauldequin, an unenhanced weapon appears to fire just as fast as the speed enhanced gun.
This means that adding speed enhancing gems to any weapon in the "Fastest" category, even if it is not quite as fast as the the fast rifles, is not really worth it. Damage enhancing gems are a better bet, or possibly +life leech gems if the weapon doesn't have any. Note, however, that too much
life leech on a 'fastest' weapon is likely to be wasteful.
(Most 'Fastest' weapons have a speed value of 0.7, a handful are 0.6.)