Many of the 3hp cyclones have 12"/300mm impellers, the same as the 2hp models. The reason the 3hp moves more air than the 2hp is because the 3s come with two filters which allow more airflow. 3hp DC's with bigger impellers would be the thing to look for if not working with something already owned.
Pete
This was my feeling too after looking at mine last week. I suspect that the 3hp motor is just put onto the exact same everything as the 2hp and sold based on larger made up numbers, I'm not even sure that the outlet from the housing is any larger on the 3hp machines. Mine has the Y split to go from the impeller housing outlet to the filters, but the outlet from the impeller housing is the exact same size as each of the 2 outlets from the Y (178cm2 area), suggesting that there should be no reason to not just stack 2 filters tall rather than have them side by side in the fashion they come in. A 150mm diameter pipe would have an area of 176cm2. The 100mm inlet adapters that come with the machines (and look identical to the one seen in Mikes motor link above) have just 78.5cm2 area.
I know Pentz replaced his 300mm impeller with something nearer 375mm as after taking measurements the motor could handle it. I have to think mine is the same as it's only drawing a little over 1200w but I would need to do more checking before even attempting to make an impeller and housing like Mattius or others have done. I do at least have that little book about making your own!
I am also under the impression that impellers also perform better when they are 2 disks with blades in between rather than how they typically come which is just one disk and blades.
On mine, the 300x90 impeller with the big opening removed to reveal a 305mm opening still only draws 1500w and that number can only go down as resistance is added.
I did see a youtube video just today about a chap replaing his 250mm harbour freight impeller with a 300mm rikon one and he saw 20% increases in air flow and 38% increase in static pressure. Though the oddity was that he didn't have to change the housing, so the old one was undersized for its housing.