After a bad string of PUGs I performed some tests. My numbers may be wrong, someone more familiar with the games' mechanics can check them, but my tests were consistent whether Phasic Rounds was used or not.
Setup: Human Adept
http://narida.pytalh...0@0@A@@@0@0@0@0 on Bronze with Phasic Rounds III and no ammo bonus. These tests show that Phasic Rounds is probably working correctly.
Warp (3):
581.25
Shockwave (6):
480
BE (9):
100*((9-2)*.15+1)*1.5(Bronze)*2(Armor)*1.65(Shockwave 5a)=1014.75
Total damage per Warp + Shockwave + BE Cycle:
1014.75+581.25+480=2076
Bronze Atlas armor: 9000
It took five attack cycles (jive 10380 total predicted armor damage) to kill the Atlas after depleting the shield regardless of ammo bonus. Both setups were tested twice, each with the same result.
If there is a damage reduction, it is less than 13.3% based on this test.
A second test was conducted using only warp to inflict damage on the Atlas' armor. Warp deals 581.25 damage, therefore it would require 16 casts to destroy the Atlas. In game tests matched this prediction. Warp's DoT was allowed to run the full ten seconds near the end of armor to ensure that the Atlas was not overkilled. Again no difference was observed regardless of whether Phasic Rounds was used. Both setups were tested twice, each with the same result.
This test would require a damage reduction of less than 3.3%.
It's possible that there is something that wasn't tested for causing this (difficulty level, two detonation bonuses, non-host etc.), but Phasic Rounds use appears to perform no different from its non-use regarding powers.