Cast vs. Forged

Forged are ultimatly stronger-however the difference to the average rider isnt great, and as long as you warm up the engine properly and run it in,and replace at resonable intervals you will get good service from any make of piston.Std pistons on jap bikes are cast so that arguement is null.

Also off the shelve forged pistons are sometimes heavier than the stock piston, hence a reduction in performance. Good quality forged are usually lighter due to a lot more machinging and are therefore dearer. The only reason to go to forged is if the standard piston is not strong enough due to poor design (very rare in Jap bikes) or if the combustion pressures are high enough to break the piston(turbos)
The other reason i guess is when there isn't a cast piston available such as big bore kits or raising the compression