Hypereutectic pistons (16 - 18% silicon) allow 1) running tighter piston to wall clearances and 2) harder aluminum alloy for better wear life. Piston ring life is also extended due to less rocking in the ring grooves. Hypereutectic pistons are mainly for normally aspirated engines (non-boosted).
The SRP line of JE pistons forged pistons does use a 4032 aluminum alloy with a higher silicon content of 11%. The SRP forged pistons run 0.025" to 0.035" clearance compared to the hypereutectic clearances of 0.0015" to 0.002". The standard low silicon forged pistons run clearances of 0.0035" or greater.
http://www.jepistons.com/dept/tech/dl/piston_instrc4032.pdf
