Is there a hierarchy of drives relating to performance?

If you mean the inexpensive Merc stuff, it is roughly
Alpha
Bravo
Bravo varieties
Bravo with stronger parts from folks like IMCO

after those you move to drives with transmissions
TRS
Speedmaster #3, #4, #5 in the older varieties
Speedmaster #6 and M8 in the newer varieties
then non Mercury drives like Arneson shaft drives and others

How much hp?

And the NXT

I'd guess an order that looks something like this
Alpha
Bravo
Bravo XR
Imco SC/max machine worx
TRS
Imco SCX/B-Max
Speedmaster 3, 4, 5, 6, 8
then probably Arneson