I do not think cumshot speed can be attributed to breed. Cum amount will rise with the dog's size. The intensity of cum shots and amount of it depends more on how horny the dog is which I know from experience. So get good at arousing your dog in foreplay.
Deciding which dog to get based on his sexual performance is dumb for a few reasons.
1. Not very dog is into sex with people. It is rare, but you have to count with that your dog might not like having sex with you. What are you going to do?
2. You should get a dog that fits your lifestyle. A large dog will have reduced lifespan (about 7 years, are you prepared to deal with that?), will require significantly more money to feed and take care of and will require more work to be put into socialization and obedience training and longer walks.
A highly active dog like a border collie will require tons of enrichment and interaction. A GSD of the show dog lineage will likely suffer from joint problems due to the fact that people damaged the breed. Do you want to support that?
And so on. Choosing a breed is not a matter of simple dick size comparison.