It's not that the drug is different in boxers, it's that boxers are different. They don't tend to show their heart disease until they collapse. So you are correct that Acepromazine does not "do anything different to boxers." BUT
boxers are more likely that other breeds to have an underlying cardiomyopathy making them more sensitive to the effects of acepromazine.
Boxers and Dobermans have a MUCH higher incidence of occult cardiomyopathy than other breeds:
http://www3.interscience.wiley.com/journal/121527251/abstract?CRETRY=1&SRETRY=0
"Retrospective report on the signs of dilated cardiomyopathy in 369 dogs in the UK:
History & Signalment of the dogs seen at a referral cardiology clinic
- purebred dog, 365 of the 369 dogs
- breeds with over 20 dogs of the breed represented
-
Doberman pinscher, 59 of the 369 dogs (5:1 male to sex:F ratio)
- peak incidence at 7 years of age (most were 5-10 years of age)
- boxer, 53 dogs (3:1 male to sex:F ratio)
- peak incidences at 6 and 9 years of age
- great Dane, 38 (6:1 male to sex:F ratio)
- cocker spaniel, 30 (3:1 male to sex:F ratio)
- German shepherd dog, 24 (6:1 male to sex:F ratio)
- St Bernard, 20 (5:1 male to sex:F ratio)
- Labrador retriever, 20 (3:1 male to sex:F ratio)
- medium and large breeds dogs over 15 kg, 95% of the dogs
- median weight of all dogs: 37 kg
- median age at presentation: 6.67 years of age
- male, 73% (2.7:1 male to sex:F ratio)"
When boxers have cardiomyopathy, their first sign is OFTEN syncope and collapse (sometimes death), and this is before anyone knew they had a problem.
"
syncope/collapse, 26%
- 70% of the boxers presented with this sign"
The incidence of VPCs in boxers with cardiomyopathy (which again, no one knows they even have at the time they present) is quite high--higher than other breeds:
"ECG, available in 94.6% of the cases
- arrhythmia, 89% of the 369 dogs
- atrial fibrillation, 49% (most common in giant and large breed dogs)
-
ventricular premature complexes, 31% (most common in boxers and Dobes)
- supraventricular premature complexes, 9%
Unless you are running ECGs on all boxers prior to induction of anesthesia or administration of acepromazine, you won't be likely to pick it up:
"Laboratory findings: mean and medians for all dogs were within normal limits"
So you take a breed with a known pre-disposition to occult cardiomyopathy leading to syncopy and collapse and give a drug known to cause bradycardia and hypotension, and you could kill the dog. From Plumb's:
"Acepromazine's effect on blood pressure (hypotension) is well described and an important consideration in therapy. This effect is thought to be mediated by both central mechanisms and through the alpha-adrenergic actions of the drug. Cardiovascular collapse (secondary to bradycardia and hypotension) has been described in all major species. Dogs may be more sensitive to these effects than other animals."
Ace has no reversal agent.
Granted, in a boxer without cardiomyopathy, the risk is no different than any other dog. The problem with boxers is that we often don't know they have cardiomyopathy until they collapse. Since there are alternatives for pre-medication, why take the gamble?