I don't know that I agree with that simply because we really don't know with any certainty that the weapon speed ISN'T a factor. It may not be, it may, we just don't know so I don't think we can simply dismiss it. My experience has been that my priest's heals seem to cast faster with the 2.6 wand & shield over the 3.6 staff. Are they really any faster, tbh again IDK.
As the the other formulas Cike mentions, I'm sure they can come fairly close but here's my point: What are people asking for the formulas for anyway? If it's to check damage output the scruitinizer works fine. If it's to tweak gear through statting, etc. then having a formula that is not correct even by a little is just as much of a shot in the dark as an experienced best guess.
Not trying to rain on any parades here, I just think focusing on finding a formula, something that has been elusive for years now, is less helpful than discussing the individual merits of known variables and proposing a more general strategy for basically every combo. For example, we can easily find out how much mdef a current boss has from Rom-Welten and can make a reasonable estimate of how much matk to stack (buffed values) to get over any mdef hurdles.
Just my 2 cents on this. I've seen a few of these posts (probably made one myself one time or another since I came from a game where this information was actually known) and they never really get any traction. Anyway, good luck with the topic if you chose to continue this discussion, I think I've said all I really have to say on the matter