"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.