Since electricity travels at almost the speed of light in wires, you truly never really need the same length of wires anywhere.
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Speed has nothing to do with it. Equal length speaker cables are suggested for the same reason one needs to use larger wire for longer AC runs...resistance. Twice the length of wire has twice the resistance. Don't believe me? Connect 200 feet of wire to one speaker and 2 feet to the other, then try to convince yourself that you can't hear a difference. Output will be noticeably reduced from the speaker connected to 200 feet of wire. That said, with a more reasonable difference in cable length (like 10 feet to one speaker and 8 feet to the other) and decent quality speaker wire, anyone would be hard pressed to ever hear a difference. Of course, everyone knows what insane perfectionists we all are (grin).
"For left and right front channels, using excellent quality cabling, would you hear a difference between 1.5m for one side and 5m for the other?"
My opinion is 'no'. I'm running 4- and 9-foot lengths of premium cables to my L/R speakers. The one clear result of matching lengths is bad--the speaker with the longer-than-necessary cable has looser bass because it's seeing a much lower damping factor.