You are correct in assuming heavier and more aggressive tires will rob horsepower, because they will. It depends on your terrain as far as which tire would be best. Here in New England, the OEM 2-ply tires or the closest thing you can find to them (Cheng Shin knobbies) are by far best. It is extremely rocky and rough where we ride and our tires are half of our suspension. A 4 or 6 ply tire is not going to have enough flex for us here to ride on comfortably. We all have rear suspension here too, but you do not. Therefore your tires are your entire suspension, not just half. If it's smooth enough where you ride that you can afford to lose the suspension 2 ply tires offer and you actually need more aggressive tires where you ride, then the meaty hard tires are for you. If you're getting them just because they look cool or something, they might bother your lower back depending on how the terrain is where you ride.
85 Tri-Zinger 60
85 ATC250SX
86 ATC250SX
87 ATC250SX
02 XR650L conversion
84 ATC 480R