6.6 has 131" waterlift and 139 CFM ( airflow)
The 1500w 5.7 three stage has 137 " waterlift and 100 CFM
Another rating is airwatts , lift x CFM/ 8.5
The 6.6 has 620 airwatts The 5.7 has about 450
So in a machine with a parallel configuration ..
you could say a triplevac 5.7machine and a twin 6.6 machine will be around the same .
A standard twinvac 5.7 machine will have 137" lift and 200 cfm
triplevac 5.7 will have 137 " lift and around 290 CFM
Twin 6.6 machine will have 131 " lift and around 290 CFM
There are more powerfull 5.7s available in the states , but none reaching the UK at the moment .