IDK, last time I did it (tried it on a creep), it instantly kills my unit... but that was a long time ago...
and based on observations (by other people), it doesn't go down in-game, only the attack animation speeds up... I tried to observe units in DotA, and it seems somehow true...
EDIT: tested it and it seems to be fixed now...
tested it using two heroes both 0.00 AC, one has 20 agi, the other 10000 agi and I dont see a problem of a MAX AS... the 10000 agi hero I used actually attacked so fast...
EDIT 2: I added another hero with 20000 agi and 0.00 AC, and it attacks faster than the 10K agi one... so I dont get the problem that you have with max AS...
Tested it with a hero both of 10,000 agi. one has 0.00 AC the other, 5.00 sec AC
and it proved that AC doesn't get lowered in-game, their animations where both fast, but the one with 0.00 AC attacked continously and the other has a 5.00 sec between each attack...
MEANING, AC will set the max attack speed that you can have...