Damage point won't effect the cooldown between attacks unless it's longer than the unit's base attack cooldown (which makes no sense anyway). Attack speed increases increase the base attack cooldown and the damage point. I wouldn't recommend setting the damage point to 0 or indeed much lower then the unit's base attack cooldown (if the base cooldown is particularly low, like under or around .5) because it'll mess up attack animations (I.E. they won't play fully, so it'll look like your unit is just stuttering instead of attacking).
The only way to make a unit attack faster than the +400 attack speed cap is to change it to another unit with a faster base attack time. This would probably be a really complex system depending on how many "steps" you want to modify a unit's attack speed by (I.E., the maximum attack speed increase between the slowest and fastest possible attack speeds). Especially if it applies to multiple units.
Another somewhat less controlled option is to simply give your units a high base attack time, but slow their attack speed by 100. This'll give you a practical range of 800 from -100 (the minimum) to +400 (the maximum) because -100 acts the same as +400. For example a unit with .3 base attack time and -100 attack speed has an attack time of 1.6 and a unit with 1.6 base attack time and +400 attack speed has an attack time of .3
The biggest problem with either method is you'll need to come up with a system to carefully control attack speed increases/decreases rather than relying on the default WC3 system. Reducing a unit's base attack time means that all further attack speed increases will be more effective, while reducing units' attack speed to -100 means that every point of attack speed will be worth more until the unit's attack speed reaches 0; not to mention abilities that reduce the unit's attack speed won't do anything.
