the nismo E16 pistons i have look nearly identical to the pistons in my E15s. however, i haven't checked them out fully yet. the nismo E16 and E15s pistons have a large inner diameter crown with a shallow dish and two small valve reliefs (as shown in the photo above). the E16 pistons have a small inner diameter crown with a deeper dish, and, IIRC, no valve reliefs due to the taller combustion chamber on the E16 (not positive on the valve relief thing, it's been years since i looked at one...). E15T pistons have a large inner diameter crown, SIMILAR BUT NOT IDENTICAL to the nismo E16 and E15s pistons. the E15T has a *1mm* higher compression height, and a deeper dish and NO VALVE RELIEFS. as i stated before, they're a little heavier to deal with excess heat.
so... you *could,* if you wanted to put turbo pistons in a N/A E15/E16. BUT you would have to REMOVE 1mm from the top of the crown (making the top ring land WEAK) and ADD VALVE RELIEFS. too much effort, in my opinion.
you also *could* put N/A pistons in an E15T. BUT you would lower the compression ratio a bit (i'm not doing the math, it's not worth the effort), and you would have WEAKER pistons.
TEE KOO, have you identified your pistons yet? the top of the crown may be larger simply because you have over-sized pistons (.040 over is pretty big...). is your engine from something OTHER than a north american pulsar? that may also explain it...