Alright, I only was able to get on a this late, but here is what I found:
At level 18 and nothing but a Triforce, Udyr's next basic attack will do 225 bonus physical damage. With five stacks of his passive, this is boosted to 293 bonus physical damage.
At level 18, each Sterak's Gage gives you +65 base AD. This means that with five, you next basic attack does 563 bonus physical damage. At this point, Udyr has 4527 health and 322 AD (numbers may vary depending on runes). With five stacks of your passive, your next basic attack does 631 bonus physical damage.
If you only have four, your next basic attack does 496 bonus physical damage, or 563 at max stacks.
Appearantly they stack with each other, so if you have four, each Gage only gives you +57 bAD.
Now, when the passive triggers, a whole new level of Triforce goodness unlocks.
With four Gages, the bonus is exactly like if you had a fifth Gage, so see above. With Five and the passive procced, Udyr has 356 AD, +73 bAD from each Gage, and 577 bonus physical damage on the next basic attack, which increases to a grand total of 699 with five stacks of your passive.
That was fun.
-Nixtarma
