To round out the discussion - it's not that the hypers are 'weaker' than forged pistons. In fact, if you do a google search, you can come up with all sorts of info -- I found tests which actually show higher tensile and fatigue strengths and greater stiffness with hypereutectics compared to forged. Both are aluminum alloys - the hypers have a bit more silicon content which, among other things, makes them more thermally stable. Clearances can be tighter and they are less noisy than forged slugs. Here's a blurb from KB about some of the advantages of hypers:
"The KB T6 hypereutectics are considerably different than the forgings. The KB pistons have shown improvement in power, fuel economy, cylinder sealing, service life, and cost effectiveness. The reduced thermal expansion rate allows the piston to be run with reduced clearance. A tight piston is less likely to rock, make noise, and burn oil. A rocking piston wears rings and increases blow-bye. The close fit of the KB piston allows the piston rings to truly seal, minimizing blow-by.
The design flexibility enjoyed by the KB series of pistons has an advantage over present day forging practices. The die for a forged piston must be designed so it can be easily removed. This limitation makes it difficult to make a light weight piston without sacrificing strength.
The KB pistons' utilization of the permanent mold with multiple die parts allows undercut areas above the pin hole and material distribution in the skirt area that stiffen the entire piston unit. The forged piston requires thick skirts to achieve comparable piston rigidity. A rigid piston rocks less in the cylinder and improves ring seal.
The forged pistons' thick skirts add weight. The design of KB pistons gives us the option to build the lightest pistons on the market. "
However, hypereutectic cast pistons are less ductile than forged pistons. A bit of an oversimplification - but you might say forged pistons have more bend before they break compared to hypers. The nature of a hypereutectic piston failure when put in a destructive detonation environment is usually more catastrophic than a forged slug. Consequently, you have companies like KB with things like this posted at their site: "Please Note: Our 40 yrs. experience has shown us that these pistons should NEVER be used with Nitrous, super charger or turbo's. They should also NOT be used with Stroker Applications because of increased piston speeds. A forged piston is recommended in these applications, remember hypereutectic pistons are still cast pistons."
However, many have used them with mild boost and nitrous. As others have said, tune seems to be the key. And if it were me, I wouldn't leave it to plug reading - I'd want some dyno data with wide-band a/f ratios. You might try a different titled post and ask those running safe nitrous tunes what kind of a/f ratios, compression ratios, and total timing are "safe" for a hyper set up.