Bullitt - the point I (initially) and Darren were making was, the speakers themselves don't have to be the same distance away (though this obviously helps!). The point was, no matter how far the front speakers are from the amp, get the same length cable and no matter how far the rear speakers are from the amp, get the same length cable (i.e. go with the furthest distance). As Darren says, this is due to the resistance of the cable affecting the sound.
So in your case, get 2x 4.2m cables for the fronts and 2x 7.5m cables for the rears. Just follow Darren's advice concerning excess cable i.e. on your front speaker which is only 2.5m from the amp, you've got nearly 2m of cable which the needs to go somewhere. Don't coil this up, but tie it in an S-curve to ensure you don't get the hum Darren speaks of. Same goes for the rear speaker which is only 5.5m - again 2m of cable which must be curved in the way Darren describes.
Hope this helps!