Return grilles as supply registers

I’m a longtime GBA reader and havent seen this question addressed before.

In my quest to increase air flow from older ductwork and a retrofitted variable-speed heat pump and two-stage air handler (which almost exclusively runs at low stage) to distant bedrooms, I am planning to test out return grilles in place of the normal louvered supply registers and wondering if others have done the same. I’ve already sealed the ducts where they are accessible in both the basement and in the bedrooms, turned down the dampers in the living room where the t-stat is located, and insulated and air sealed both basement and attic to a significant extent. The bedrooms still stay a little too cold in winter and a little too hot in summer for my liking and quest for perfection, and before I add Envi wall heaters or even minisplits to them I’d like to exhaust all simple and cheap options.

My understanding is that supply register louvers add a bit of turbulence and therefore help with the conditioned air mixing with its surroundings. They also allow for directionality of airflow. Conversely, the addition of more surface (blocking) area and more turbulence to a single smallish (4 x 10″) register in a medium sized distant bedroom is realistically increasing the static pressure and subjectively feels like its blocking a lot of airflow that I feel when I remove the register. 4 x 10″ return grilles arent carried at the big box stores but easy to find online, and have less surface/blocking area and turbulence-inducing physical features for sure.

I have seen reference to the fact that without proper mixing and turbulence, heated air would just rise to the ceiling unimpeded and you’d end up with a heavily stratified room temperature. It seems to me though that the warm (not hot) air from a heat pump would be less likely to beeline so rapidly, and with longer run times from a heat pump there would be more mixing opportunity regardless. 

I’ve also seen the purported value of user-setting directionality in air flow, which I dont totally buy especially given the relatively low velocity of the forced air at low stage. We have 14 supply registers in our house and I’ve never benefitted from directing any of them to flow in any direction in particular. 

Anyways, this will be a cheap and easy test for me as a return grille doesnt cost much. I might try to test the change in temp but wont be measuring delta in CFM (if any). I know I’m sort of grasping at straws here but even a minor change in air flow to a decently well insulated bedroom would seem to me to have a big impact on comfort.

As an aside, I have already implemented a similar test on an underperforming wall register at the base of a staircase and cathedral ceiling (and even mounted it upside down to match the direction of the flow from the duct), and quite honestly it seems to be making a significant difference in both air flow and temperature throughout the vertical span of the staircase, as silly as it may sound or look.

What am I missing, GBA experts?

Thanks,
Evan