Half the length, or double the quality?

Only deal I can think of that would double the needed length of speaker cable is going from mono blocks to a stereo amp. But that doesn't seem to be the case. So what exactly are you doing?

The brand of cable would help. 

You might like the change.  I think gaming this out without auditioning it is not the right way to go.  Get the 8' and listen. If you like the sound, leave them as-is, or trad down for shorter.

Whom do you expect to be able to tell you whether the move is lateral, especially since you provide no information regarding brand and model? It doesn't matter; they� couldn't tell you anyway, and there would be no consensus. 

This post is the equivalent of, "Will I like this new car better than the other?" I don't know, you tell me! 

IF the 8' is better and you have bi-wired speakers, cut down the 8' some to make jumpers. IF the speaker is single wired, cut the 8' lengths in half to double up the wire. Almost assured big improvement either way. Likely, you will end up with superior sound, even if the 8' cable was not absolutely superior initially.  

Twice the length means twice as much phlogiston! NG!

easier to sell an eight inch pair if you ever want to upgrade than a 4 inch pair.

OK, so I must confess. This was more of a hypothetical question. I was just wondering how much better a speaker cable would have to be if it were twice as long as needed. I have appreciated Erik's questions in the past on non-specific product topics. So, generally speaking, does doubling the needed length (with a better cable) come out to about the same. Keep in mind you can't cut the better cable in half, and therefore have your cake and eat it too.
I'm just trying to "poll" the audience. Sorry for any confusion, and this post may actually make it even more confusing.

Mapleshade believes 8 ft is the ideal length for speaker cables. Some others also believe that shorter is not always better. Of course no agreement. 

Maybe.