I did some testing today to see how buffs stuck and their limits. It appears that buffs are additive rather than multiplicative, so the formula for speed, for example, would be:
Speed = Natural Speed + [Natural Speed * Buff One] + [Natural Speed * Buff Two] + ...
To give out some Unclean Beast numbers for reference:
Start Speed: 6.3
Speed w/ Skills: 6.9
Speed / Skills & All Passive Items: 11.7
Speed Cap: 14.0
It appears the Attack Rate cap bottoms out at 0.50 seconds per attack for Unclean Beast, if not all heroes.
Has anyone else done similar testing, and if so, have I missed anything? I'll continue to look into this, as I want to know if LifeSteal stacks, but that one will be a bit tougher to gauge.