Before the changes on PBE, Yi would need 225 total AD to deal as much true damage as Irelia does on-hit. She does 75 true damage on-hit at Hiten Style's max rank. To be able to deal as much true damage on-hit, Yi needs 225 total AD and to max Wuju Style. This isn't completely difficult to get with only a few AD items, however; generally, a Guinsoo is enough. Considering that Yi tends to get a lot more attack speed items and that he has a natural AS steroid that also makes him immune to AS slows, Master Yi does more true damage on-hit than Irelia, even if he has a lower value.
20% of 225 is 45. 45 (the scaling) + 30 (the base damage) = 75 on-hit true damage which is the amount of true damage that Irelia does on-hit with her Hiten Style maxed.
However, with the new PBE changes, Yi will need 175 bonus AD to be able to deal as much on-hit true damage as Irelia. This is taking into account the changes to the base damage. Because base AD does not need to get into calculations now, Yi would need to get a lot more AD items to be able to deal as much true damage. Guinsoo is not enough, even when fully stacked. Not even with Guinsoo + BotRK, Yi is able to get to the old values. The only way that he can get to the old values is probably with a dedicated full damage build.