You can ask any MOPAR gearhead. While the power output of the Gen3 HEMI 5.7-liter and 6.1-liter is nothing to scoff at, the motor will respond well to specific valvetrain upgrades.
For example, the stock beehive valve springs in the 5.7 and 6.2-liter HEMI are excellent for stock applications. But after bolting in a set of camshafts with a more aggressive cam profile, the stock OEM valve springs are not enough. In this case, upgrading the valve springs are a good idea.
After considering the valve springs, the stock valves should be considered as well. A set of aftermarket or custom valves with lower valve lock grooves will move the retainer position downwards to accommodate larger-diameter straight-type valve springs. The 5.7-liter can also make use of custom valves to improve flow and horsepower. The trick is to swap the stock 2.00/1.55-inch valves from the 5.7-liter to a set of 2.08/1.600-inch valves derived from the 6.1-liter HEMI. This alone will enable the 5.7-liter HEMI to benefit from a 7-percent improvement in flow to improve performance.
The cylinder head in the 6.1-liter HEMI has larger rectangular ports, which allows 20-percent more port volume compared to the cylinder head in a 5.7-liter (with smaller and nearly square port openings). Swapping the cylinder heads in this regards (and swapping the valves) will enable your 5.7-liter Challenger to produce more horsepower with better throttle response.
The good news is both the cylinder head castings in the 5.7 and 6.1-liter HEMI share the same bolt spacing, external thread accessory positions, valve angle geometry, and cast spark plug wells. As with all engine rebuilding projects, it is always best to seek the recommendations of a professional engine builder.