The alacrity was handled in City of Heroes by allowing up to 3 stacks. One stack I think provided 33% recharge. each additional stack added more, but not 33% more each. I suggest allowing up to 2 or 3 stacks of alacrity which can provide at max 66%. It should be 50% for 1, 58% for 2, and 66% for 3.
I honestly think alacrity should be no less than 50% to make it a worthy buff. In my pic attachment I depict when there are 2 stacks of alacrity it grants 58% cooldown rate, and 3 stacks grants 66% cooldown rate. The numbers can be played with but I think at some point we should be able to reach 66% alacrity, even if for 2-3 seconds, and the rest whatever pleases ANet.
I had to hand draw it out because MS paint was becoming too difficult :[
