You might try using one half of each twisted pair for the hot signal and the other half for the ground. This will increase capacitance a bit but it will lower inductance, which is the real problem at these currents and impedances.
Note that a tighter twist means more copper in series with the speaker, not more copper in thickness. This will increase the series resistance a tad, which will not be a problem if you are using all 27 pairs. The best benefit of the tighter twist is less inductance, which is a very big plus.
Don't worry about time smear; you can measure this and you will need to have 100 feet of speaker cable to have 1/4 inch of time smear. With the twisted pairs you are using, I doubt you would even have that.
Note that a tighter twist means more copper in series with the speaker, not more copper in thickness. This will increase the series resistance a tad, which will not be a problem if you are using all 27 pairs. The best benefit of the tighter twist is less inductance, which is a very big plus.
Don't worry about time smear; you can measure this and you will need to have 100 feet of speaker cable to have 1/4 inch of time smear. With the twisted pairs you are using, I doubt you would even have that.