Don't worry, I'm an engineer!

"Does it cost more pollution to create high octane gas?"

No, there's only a small amount of leeway in how a refinery fractionates its crude. Gasoline itself is a blend of different compounds.

One desirable characteristic of gasoline is how much it can be compressed before it auto-ignites (you don't want this to happen because it'll throw your engine timing off) and a specific band of compounds (known as the B-T-X fraction: benzene, toluene, xylenes) increases this property.

The reason high-octane fuels are more expensive is because there just isn't enough of this stuff to go around. You get your crude and when you separate it into components there is x amount of stuff you can make gasoline with and y amount of B-T-X additives where y is less than x. Supply and demand economics.

"Does it release more pollution when used in a car?"

This is a loaded question.

At high compression ratios, the fuel burns more efficiently so you get more complete combustion, more power for the fuel. So in the strictest sense, no. Let's say you have a VQ35, all else being equal you will have better combustion (so you get a higher CO2 to CO ratio with C being stoichiometrically limiting) and more importantly you get more power per unit of fuel so you need to burn less of it to do the same thing.

There are a number of caveats. First you need an engine designed for high compression, otherwise low or high octane will ignite all the same and you're just wasting your money.

Secondly, high compression engines are usually high power, high performance pieces of machinery. So yes, you will get to 300hp more efficiently with a high octane fuel versus a low octane fuel, but if you are getting to places with a 300hp engine instead of a 100hp engine, you're not really saving any fuel are you?

The corollary is that engines have very different sets of performance envelopes so what's most fuel efficient depends on your driving habits. If you do a lot of hard acceleration, a Nissan Sentra won't have enough torque to push it on the low revs, so your car is going to have to gear down and rev harder. On the other hand, a straight-6 BMW is going to be able to generate the same amount of power without breaching 3000 on the tachometer. An engine operates more efficiently in the 2000 - 3000 rpm band than, say, 5500. So in that case, yeah, you might actually save more fuel in that 300hp BMW than the 100hp Nissan. Jeremy Clarkson might be an ape but he got that right, it's more about how you drive than what you drive.

(I've simplified some things but the point is generally valid)