I have tested and used a Paco Kelly Accurizer and a Waltz die for the last 14 years.
As to increasing the precision [tightening groups] of 22/LR ammunition I've found neither tool will increase the precision of top end match 22LR. Use of the tool can be beneficial to precision with the medium priced "match" ammunition. It really improves the precision of some Winchester "Match" [White Box] I have and also gives a bit of improvement to CCIs "Green Box" match 22LR. The use of the tool will many times be most beneficial to bulk 22LR as it doesn't always reduce the size of most shots but most often eliminates the fliers. You have to do some experimenting on how much modification/swaging gives the best results. I don't use a mallet with the Paco Tool but prefer to use a press. I use the Lee hand press with the Waltz die.
One of the biggest improvements for me is increasing the terminal effect. Using the "Nasty Nose" or HP tool can really increase the terminal effect on small game/vermin. You have to see or hear [the "whop" of the bullet hitting a small ground squirrel] to believe just how much it increases the terminal effect. CCI Blazer is my favorite to do either on. The soft plain lead bullet is devastating on squirrels and rabbits. I also re-HP most bulk copper coated 22LR HP HVs [Federals and Winchesters] swaging the bullet to a uniform .225 and opening the HP to the max just before the copper coating begins to split. They still feed fine out of the magazines in my bolt actions, my 10/22s, my M261 in my AR and in my semi-auto handguns.
Keep in mind that either tool will not make a silk purse out of a sow's ear because some of the bulk 22LR ammunition that was produced during the Obama shortage years is beyond help.
Some modified [before and after] with the Paco Kelly Accurizer;
Attachment 255596
Some CCI Blazer modified [before and after] with the Waltz die to HPs with small HP, medium HP and large HP;
Attachment 255597