I ask this same question to various cable manufacturers, many times in the last 10 years. The general rule seems to be that if you cannot cut the length to half of the original, the change is not worthwhile. Same goes with increase, doubling the length can result in loss of performance, adding slightly is usually not audible. Of course there are situations where the length is excessive (30 feet?) where you damage the signal to the extent that the rules are no longer accurate. The precise answer to your question would be that shortening it to the 8 foot length would be better, and any less radical change would probably not be worth the effort. By the way, this formula has proven to be accurate in my system over the years.
Speaker Cable Length
I know that this thread has been visited before in gerneral terms, but I would appreciate feedback relative to my specific situation. My current speaker cables are 15 feet each. What, if any benefit, will result in using 10-12 feet lengths? What about 8 feet (the shortest I might be able to go after extensive re-arranging of equipment)? Thanks in advance.