ok PDL trait (tested tier 1 and 2 with cor with subs, tier 3 with monk with Aureole and tier 5 with DRK with Aureole. Aureole has low base damage so it's still possible that some rounding could mess this by some margin). Also do numbers seen in chat are rounded up?
Gun
Bee tier 0: 1706
Bee tier 1: 1755 (+2.8722157%)
Bee tier 2: 1802 (+5.6271981%)
Gun
Worm tier 0: 1365
Worm tier 0: 1404 (+2.8571429%)
Worm tier 0: 1442 (+5.6410256%)
Aureole
Bee tier 0: 202
Bee tier 3: 221 (+9.4059406%)
Bee tier 5: 233 (+15.3465347%)
Aureole
Worm tier 0: 162
Worm tier 3: 177 (+9.2592593%)
Worm tier 5: 187 (+15.4320988%)
For me it looks like it might be like this:
Tier 1: 2.8%
Tier 2: 5.6% (+2.8%)
Tier 3: 8.8% (+3.2%)
Tier 4: ??? quesing 12% (+3.2%)
Tier 5: 15.2% (+3.2%)