I wonder if there is something here about hybrid vigour? Collies apart, the cleverest dogs I've come across have been mongrels of one sort or another. That said, I have a Jack Russell who displays real signs of intelligence. He's a working dog, a famous ratter in our area, and a genuine assassin who also catches a lot of rabbits (we're overrun with Bugs and his friends round here!). He's not really fast enough over the ground to catch an adult rabbit, but this is where the intelligence comes in. He's worked out that a rabbit will run a short distance, then jink either right or left when pursued. He actively gambles; he anticipates the change of direction, and jinks just before the rabbit does. If he guesses wrong the rabbit gets away. If not, RIP Bugs.
He's also really good at obedience and agility classes, regularly winning prizes. (Although he won't do the 'little dogs' agility course, because like all Russells he considers himself a 'big dog'). I'm a psychologist by training, and he seems to demonstrate some cognitive ability when compared to my other dog, a Lakeland terrier who is as daft as a box of frogs. I suppose the question is always 'how much is breeding, how much is instinct, how much is handler-induced and how much is actual 'thinking'?