If Flash has roughly 600 PD late-game, then you find yourself in a situation where the burst from the modded Psi will deal 350 PD. But this doesn't proc often enough to make it worth it.
The damage from the Blue Scarab with the +20% mod will actually be more bursty when used in conjuction with a W, while dealing much more overall sustain. At the same 600 PD, when using a W, you will deal an extra 130*4=520 PD worth of burst. Additionally, ANY auto-attack will have an extra 130 PD added to it, and with even just flat pen, you will really hurt people.