Where did you get the info that the +15% DW bonus was addititve with gear bonuses (+%IAS and +APS)? That seems rather counterintuitive to me, and would make dual wield quite a bit weaker if you had a lot of IAS gear.
As far as +APS versus +%IAS, it would be nice to see how these two interact with each other. I agree that additive makes more sense than multiplicative, but you can't assume anything, we need data!
It would also be interesting to see whether mainhand +%IAS/APS could affect your offhand weapon (or vice versa). In D2, IAS on a weapon only affected that weapon. (as opposed to +IAS on gloves and armor, which would affect both your weapons) However, Haste in WoW is the same regardless of source - 50 Haste rating on your mainhand will increase attackspeed on both MH and OH.
If IAS/APS work like Diablo 2, things become complicated because we actually have 4 different sources of attack speed (per weapon):
1) Weapon IAS
2) Weapon APS
3) Armor IAS
4) Armor APS
And it is concievable that any of the four could add or multiply in unpredictable ways.
It also suggests that, at least up to 1.80 APS, there are no diminishing returns. There could easily be diminishing returns at higher APS, because I really have a hard time imagining 10+ APS (that is faster than most machine guns in shooters, ~600 RPM = 10 APS).