Firstly, anyone who is opposed to the genetic modification of pets on moral grounds, must also be opposed to selective breeding of pedigree pets... otherwise they're simply being hypocritical.
I'm opposed to both. Not in principle, of course, but because the use such techniques are routinely put to (and will be put to in the future) are clearly harmful and immoral.
Through selective breeding for arbitrary, idiotic purposes, we have produced dogs that are selected for their facial disfigurement, to such an extent that they cannot be born without human intervention. To such an extent that they cannot adequately breathe.
We have produced cats prone to blindness, simply to gratify our desire for a certain eye-colour.
We have produced tiny toy dogs, lapdogs, who have short life-spans, erratic temperaments and fragile bone structures.
And now, we are attempting to produce a cat which people may not be as allergic to. Whoop-te-do. Exactly what does this do for cat-kind? Many cats are thrown out and live ferally throughout the western world. What chance would normal stray cats have to be adopted by families, when GM anti-allergy cats are brought into the mix?
In addition, how many failed experiments were there, along the way, how many cats born with genetic flaws resulting in congenital defects? It would be interesting to find out how many creatures they had to euthanise during their wondrous search for the better... better housecat.
Nope, I have nothing against the exercise in principle. But this usage it's been put to (and probably ALL uses we humans will put the science to in the future) is futile, self-aggrandising and immoral.
Now, beep me when they've created a genetically modified human baby that is incapable of being allergic to cats. Then I'll join the parade, thank you very much.