If you have the typical builder grade ceiling/sidewall registers, replacing those with commercial models will make a huge improvement. Comfort is better because air can be more precisely directed, and pressure drop is lower reducing duct leakage. I highly recommend register upgrades to those who can afford to shell out the approximately $20 per register. 4 things I've found when using commercial registers for the ceiling:
1: Do not try to use the 821's to control air volume, you will need to upgrade to a higher end register if you want volume control. IMHO registers shouldn't be used for volume control anyways, it should be done by duct dampers
2: In areas with significant heating degree days point registers DOWN to get the warm air to the floor. This also helps with all the warm air not collecting at the ceiling like you have with builder grade registers.
3: Since these registers have so much more free area throw is affected if you try to make the air go multiple ways. I've found they perform best when throw is in ONE direction.
4: One you have installed these in your house you won't ever be happy with builder grade registers again. Plan on buying more commercial registers for your next house. Yes, they are THAT good.
Commercial registers, I use the 821 series for my house:
How'd you get interested and or learn about Hart & Cooley?
I wasn't happy with the 4 way registers that were installed in my home when I bought it. It made no sense to have heat and AC blowing 4 directions when the registers were 2' from the wall/corners. One bedroom had a 4 way on the SIDEWALL, conditioned air was directed at the ceiling!! A trip to the local HVAC supply house and I found the 821 series registers, bought one for the sidewall and liked it. Replaced the others soon thereafter. Seriously, you need to try at least one and you to will be hooked.