It's only a 4 foot run from a plenum box. The other (2) 6 in runs coming out of the plenum box feeds the upstairs hall bathroom and the "toilet room" of the master bath. The run I was thinking of changing is a 8in duct connecting to that 5X8 boot. The plenum box if connected by a 10inch duct from the main air-handler plenum.
I caulked around the boot and grille last summer after reading how necessary it was.
No, it's not noisy...but wanting to reduce the static pressure so more air will flow into the master bath. The bath has a 16-18 ft vaulted ceiling and the supply vent is located about 8 feet off the floor. The bath has a ceiling fan which helps...but my thinking was solely about increasing the volume of air.
Looked a little closer...they installed the 8in duct into an 8X6 reducer into the boot...when I take the grille off, you can feel a change in velocity...I wonder why they did it that way?
If it's a cheap stamped steel supply register grill you removed, no surprise the airflow improved. Get a curved blade register, they are considerably less restrictive. You may not need to do anything else but that to improve air delivery to the room.
If you're lucky your local home center may have them. They can be identified by the curved blades that are movable...the fixed blade type, although curved, are less useful, IMO.
The curved blade models come in one or more directional "throws". Optimum "throw" from a supply register if it is near an inside wall (and mounted to the ceiling) is to "throw" the air stream toward an outside wall. The airstream should not point sharply down toward the floor...that will create uncomfortable drafts.
If you have high sidewall supply outlets, adjustable blade grills (not curved) are much better than cheap stamped steel two or three way grills. The damper inside the register can be used to aim the airstream slightly toward the ceiling, improving the throw of the register year round. Again, the object is to allow the air to become thoroughly mixed with the air in the room before it reaches the areas of the room where you normally occupy...oddly enough called the "occupied zone".
Here is another interesting tidbit...When I pulled the grille off, being 7ft high on a ladder, a could feel a cool breeze(granted not much) but it was there...with it being a little breezy outside...can it be entering from the exhaust on the roof and going thru the ductwork...and if so, how do you stop back-drafting from the exhaust flue?