There is little difference between frozen pipes at 15-degrees and frozen pipes at 25 degrees. Frozen is frozen. The expansion rate is the same for ice at either temperature.

The link I posted for Norfolk shows the low on the 4th was 20 (not 25), but the only difference between damage at 15 vs. damage at 25 is the duration of the temperature. At 15 it takes less time for things to reach the freezing point (about 28 for salt water and 32 for fresh, or somewhere in-between for brackish).

Edit: I agree that the plastic tubing isn't the concern, it's the rigid fittings.