Oil pan environment...
Inside an oil pan at speed it's HELL. Oil is all over the place as vapor, droplets, and on some motors as a thick rope that drags on the crank spinning off the throws. Knife edging puts a bit of an aerodynamic contour to the crank (as I understand it) so that it will cut through the thick atmosphere inside the pan. Polishing up the crank will also remove many potential stress risers and allow oil to free itself most easily. Reshaping the crank in this manner will effect balance but if you're at this point you're going to have things balanced anyway so no biggie so long as strength isn't compromised. Crank scrapers, baffles, anything that can be done to keep the oil where it belongs out of the crank's way will all help.
I'm not sure that knife edging by itself will get you much horsepower but once you get to a certain point with any engine every little bit helps! If you're to the point of having everything torn apart this, like replacing rods with stronger pieces, is one of the things to consider NOW as opposed to after putting it back together and iwshing you had dome something or other that you skipped.
Hrm, any aftermarket oil pans for the Supra or has the stock pan got very good oil control from the factory? Not ever heard of one losing oil pressure on the skidpad or anything...