Although this may sound silly, do we know of anyone who has broken a piston on purpose outside of being used in an engine, like smashing it with a sledge and awl for example? Just to see how it breaks and what the grain looks like basically.
Anyway, as for how hypereuteric pistons fare under hard use, many of my racing buddies have said from experience that they are only a smidgen better that regular cast in terms of performance potential.
They make a great oem piston and wear well, but they can't stand near as much impact force as forged when it comes to things such as knock and the like.
Honestly though, for a stock Sti, they make a much better choice.
(I'm assuming that they are hyperueteric.)