Personally I find it almost laughable to say this or that shell doesn't make a difference. Everything makes a difference, 21g against 24g makes a difference, you might kid yourself that it doesn't or quite simply not be on the required level to detect that difference (not being rude) but there is no way that an extra 50 pellets don't matter. Same with 24 against 28 and most certainly from shell to shell.
The difference is not in a single shot though, no good breaking a few 85 yaaaaard crossers with something and declaring it fit for a king, regardless of shot size, load or speed - it's the overall package that makes a difference. You have to be happy with the whole package, the cost, speed, shot size and hardness, availability, pattern density, recoil, packaging even. Now I'm the first to admit that brass height is just a marketing ploy but the fact remains that manufacturers of top end clay loads will put their best blend into the dearer ones hence why most of us tend to prefer these had price not been a factor.
Cheap shells are perfectly capable of helping you clean up some very tough stands but what they can't do is maintain your confidence levels right at the top, all the time. The slightest thing can let doubts enter your mind with obvious results, this is where the extra £30 matters, better breaks, hence easier to read hence easier to repeat.