I inquired to electro about this a while ago, they sent me the excel data sheets over for the motors. The 1600w motor has 550 air watts 148.8 inches of water lift and 110.3 cfm this means a two motor version should give roughly 221 cfm and 148.8 inches of lift compared to two 6.6 motors which have roughly 133 inches of lift and 278 cfm.
Alltec still sell the 6.6 motors and the evolution was designed with these motors in mind so they may fit these on request. That's if it is not already an option like on the storm. If you look at the data sheets it appears that even though the electro has more total lift the 6.6 will provide more actual in use lift at most orifice sizes.
But what is the 'cleaning ' orifice size when the wand is in the carpet , thats what matters .
If you look at the conical version of the 1600w that starts to increase over the 6.6 at 0.5 and lower , the 1800w version at about .75
Stuart , it would be best to message Alltec and ask why they are using the Electro if they are , im sure they want to offer customers the most performance so perhaps they find they work better in reality .