It depends on your performance goals. A $50 Prometheus hard piston in a sub 400fps, low rate-of-fire build is just overkill and a waste of money. 1/5 of the value of your gun in a consumable part? haha.
Any piston will get "shredded" if not installed correctly - prometheus included. Installing a sorbopad will almost instantly fix your Angle of Engagement problem. I'm restocking these in my store later this week.
You should upgrade your piston if you're going above 400fps, 20rps or both. And even then, there are cheaper alternatives than the Prometheus. Don't go rushing off to buy parts until you know what you need. Your stock piston should last a while if you do the proper preventative maintenance.
Once you're ready to buy a new piston, read this:
http://www.airsoftstore.ca/index.php...jqgk74shqrubf7