Police car alt, questions?

I have recently bought a high out put alt., Its a 200 amp alt. I bought it thinking it would solve my problems. My problem is that I need high output at low rpm's like a police car as a example. After I have bought my alt I am told by everyone that they do not put at much if anything at idle:o they are made to put out at like 2500-3000 rpms. Should I have bought a 450 dollar alt from powermaster or are they the same, low volage at low rpm?
Police cars for example have endless electronics that have to be used at low rpms. Can i get a hold of one of their amps at the local parts supply?

After I got the alt ( pulley was smaller than stock)and was not happy with it they sent me another pulley thats even smaller. It was 1 7/8" across the face which I was told was the smallest one.