Ingress Minimum Attack Efficiency

I was experimenting with the "hold fire" option to get the 20% bonus attack this morning and I was doing it with some level 1 bursters. Nearby high level resonators (7 & 8) reported damage as -1%

I thought it was interesting that such a high level portal was damaged by a level 1 burster, so I gave a level 6 burster a try with the max +20% bonus on the hold fire, but again I only got -1% damage

I didn't think about repeating it and checking the actual XM damage from each attack, but it made me question whether due to maximum mitigation, using a lower level burster would acheive the same damage as a higher level burster.

Does anyone have any evidence of this being the case? I will experiment again but someone might already have the data available