The pent design is no more efficient than the Hemi as far as dirtiness goes for smog, it is not any more efficient, it just has the ability to purge better, thus burn better, than the Hemi. I misspoke about the Hemi design itself, the hemispherical design is used by others, the last one was a Ford 4 cylinder or V6, can't remember, but was pretty close to the old A engine design, pretty flat and curved. If not mistaken, I don't remember seeing a single domed piston inside the 5.7 or 6.1, kind of flattening out the combustion chamber was the way to keep the compression up, along with those quench areas around the sparkplugs. I have told the story many times of the guys racing the 426 when it came back in the early 60s (1964 I believe), and the guys simply hated it because they kept breaking them as compared to the 392 Hemi. Turned out the engine was much more, not less, efficient in burning(burned faster, not slower) and by simply backing the timing back to a total of 36 degrees over the 38 degrees the 392 ran, fixed the problem. Of course, the 392 did have flat topped pistons, the domes inside the 426 are what makes the dirty, along with the bore size being 4.25inches. A proper shaped piston dome is actually a key that is being missed with the burn and cleanliness of the Hemi of today, I just don't think they really knew what they were thinking by adding the side quench areas and a dish in the piston to get the compression down, not up.
Torque and hp can be built or lost through the design of the camshaft itself. The pent design requires less lift to flow the same amount of air a 5.7 needs with the cross section of the valves being opened (lifts for the 4 cylinder engines crack .331-.360 inch at best), whereas the 5.7 is closer to .480 inch, and then you start talking duration and overlap to build the power, don't need to choke the engine down with active intakes, but it is the function of VVT (as you well know), to reduce the overlap, make the valves open and closer sooner in relation to BDC and TDC to make that bottom end power, then spread apart the opening and closing to not peter out and have an engine sucking for life when it hits 4500rpm, gives the ability to extend that to 7000rpm and above, but, this has absolutely nothing to do with the pent or Hemi design, and that was proven with the Viper engine with its VVT camshaft, all the videos on the track keep talking about the engine's ability to make more power the faster you take the rpm, and the videos show this to be true, and it is neither pent or Hemi.