Here's a search I did on Ace, more from a vet's perspective I think. The text below the link is from the bottom of the link's page. I found it interesting. That said, there's no way I would ever give my boxer ACE. Not worth the risk.
http://www.veterinarypartner.com/Content.plx?P=A&A=570&S=1&SourceID=52
IS THE BOXER SENSITIVE TO ACEPROMAZINE SIDE EFFECTS?
The answer is that no one knows for sure and no hard proof or scientific studies are available for review. Several anectdotal reports exist of boxers dangerously dropping their heart rates after routine doses of acepromazine.
This had generally been thought to be chiefly a problem in the U.K. for a group of genetically related dogs, but recently the University of California at Davis Veterinary School Anesthesia Service issued a memo which led more U.S. veterinarians to take the rumors more seriously. In this memo, the anesthesia service describes three individual boxers suffering adverse reactions (respiratory arrest in one and severe drop in heart rate in two) and suggests that acepromazine be considered contraindicated in this breed.
While it appears to be prudent to avoid acepromazine in the boxer, it is important to realize that acepromazine has been used widely in boxers with no problems and that adverse events are unusual.
http://www.veterinarypartner.com/Content.plx?P=A&A=570&S=1&SourceID=52
IS THE BOXER SENSITIVE TO ACEPROMAZINE SIDE EFFECTS?
The answer is that no one knows for sure and no hard proof or scientific studies are available for review. Several anectdotal reports exist of boxers dangerously dropping their heart rates after routine doses of acepromazine.
This had generally been thought to be chiefly a problem in the U.K. for a group of genetically related dogs, but recently the University of California at Davis Veterinary School Anesthesia Service issued a memo which led more U.S. veterinarians to take the rumors more seriously. In this memo, the anesthesia service describes three individual boxers suffering adverse reactions (respiratory arrest in one and severe drop in heart rate in two) and suggests that acepromazine be considered contraindicated in this breed.
While it appears to be prudent to avoid acepromazine in the boxer, it is important to realize that acepromazine has been used widely in boxers with no problems and that adverse events are unusual.