From what I understand, a ceramic coated stock cast piston will not take as much abuse as a forged piston. It will certainly lessen the risk of frying the piston under detonation vs. a plain stock piston, but in the end, it is still a stock cast piston...nothing more.
Are forged pistons necessary??? That really depends on how much juice you plan on pushing through the motor. I don't have much experience with the 5/6m's, but a stock 7mgte can run run 1bar all day and night long on a properly tuned engine. It can even handle the odd 16-18psi runs down the track...but at that level you're getting real close to the stock 7m pistons breaking point.
Sure, I know a lot of guys who have run upgraded turbo's in their stock pistoned 7mgte and are pleased as punch. Unfortunately, I know a lot more guys who have gambled that the stock pistons can handle 16++ psi daily and have lost that gamble miserably.
I tend to side on caution far more than other folks, simply because i was one of the folks in the above example who gambled and lost just like most of the mk3 guys i know who have gambled. As the saying goes, an ounce of prevention is a pound of cure...Good luck.