First, This is a supply vent? where is the closest return?

it could help a lot. One way to empirically test that, before cutting up your wall, is to put a fan in there so heat is distributed ceiling to floor. The fan should blow. pretty much, straight up to the ceiling from a floor location. If you have a box fan, point it toward the ceiling supported on an open crate, laundry basket, or something like that. Let the room come to equilibrium temp. with a thermometer on the bed or a chair. Start up the fan and run at low speed. Leave things go for an hour and then look at the thermometer again.

If that does not work, you probably need more supply or a return in there. If it does work well, your mods will probably work well. I would consider keeping both vents, high and low, and use the high one in the summer and the low in heating season.

Thanks ionized!

It's a supply vent and there's no return in the room, the closest one is out in the hall.

We'll try out your suggestion this weekend - thanks again for your help..

How does return air exit the room? Is the door undercut sufficiently or is there a grate somewhere?