NGK Iriway 8's

Hey guys, 
 
The engine from the half cut that i have recently got from Japan was running NGK Iriway 8's. The engine has had some track time and is lightly modified (bigger IC, chipped ECU and running more boost).
 
My question is; are Iriway 8's too cold for street use or would it be better to at least go one range hotter?
 
I cant help think the previous owner was using two ranges colder than stock for some reason other than a little track time.   
 
Thanks in advance.