If the goal is to achieve a theoretical 100% perfect sound, as you move upwards from within a mfg line, the starting point towards achieving this 100% is higher. The biggest killer of speaker performance is room interaction. A less expensive speaker may sound better in a given room than a more expensive speaker. Let us say that the room cuts the speaker's potential in half. This said, the most expensive speaker max out at 50% of the goal while the less expensive speaker max out at, lets say 40%.
Now for the fun part. Each component in the chain interacts with each other. Buying the best of everything does not guarantee the best sound. Lets use this analogy. A major league team can go out and sign the best players at each position, yet never win a championship. There is the lack of chemistry that comes in play. This chemistry also is applied to system matching. The infinite variables involved with matching components to maximize sound is astounding-->speakers, speaker wire, preamp, interconnects, source, amp, power cord, line conditioners, etc. This is why you cannot assemble the perfect system with one purchase. There is the much publicised trial and error, buy and selling that you see on audiogon. A true audiophile would never be satisfied and will always be searching. Are more expensive speakers better? Not necessarily, but it does give you a higher starting point from which to start your quest to the ultimate sound.