JE, CP, Arias, Ross, etc (and probably US Cosworth) use the same foundry anymore. Their base forgings are all similar. Wiseco does their own forging,but not sure that it matters.
CNC machines along with foundry consolidation have removed a good bit the "art" of making pistons and have leveled the playing field when it comes to QC...
Nowadays, it's more about the features and design of the pistons such as valve reliefs, oiling, cam, coatings, porting, rings, quench area, pin placement, skirt design (cushion, X, rigid, etc), and dome design...
Most of the aftermarket pistons are 2618, but at our power levels a forged 4032 piston is probably fine. The arias pistons we spec are 2618.
Not much to do on the 2ZZ with dome design. We use a "mini dome" on our 9:1 pistons and I saw Cosworth did the same when they came out with theirs. Mahle and wiseco just use a dish. Doubt either do much impact on flame propagation. The dome is so minimal on the 10-12:1 pistons that it probably doesn't matter...
Most of the other items do matter a good bit though...
For instance:
Valve reliefs... We found that the wiseco pistons had the valve reliefs miscut early on. through a lot of fighting with them, we got them to acknowledge their mishap and they changed their design accordingly. Terrible experience that cost us a lot of money, time, and gray hair. I won't deal with them again unless I have to
Another thing that matters is pin location and cam. I tore down a motor from a competitor that uses JE pistons in their sleeved motors. They were farming out their engine building to Nogrady, but I believe he's no longer doing their builds. Regardless, each JE pistons skirt had significant scuffing on one side of the skirts. I checked the pin location. It was not correct. I didn't check the cam, but the pin mis-location could have easily caused the issue... incorrect cam could have contributed, but the pins were absolutely in the wrong position for a 2ZZ. JE is a fine piston maker but whoever was calling the shots on those pistons was missing the boat. Those pistons had no coatings either, FWIW.
Features such as gas porting, the type of gas porting, skirt design, ceramic tops, skirt coating, add'l pin oiling, ring dimensions, etc all play into addition design criteria that can make one piston better than the other. I'd not forget the importance of dealing with a piston company that has good customer service (recalling the Wiseco BS from years back).
The alloy used is one factor, but far and away not the only or even the primary factor... I don't think it's a crime to look at a single factor and I'm sure others don't either. Forums tend to find a subject and blow it out of proportion. If you tell the piston maker the intended use, level of boost, and fuel you're running, they'll recommend the alloy. It's that simple. The other stuff I mentioned are features that cost money or specifications that are derived through experience and good measurement. Recall the JE and Wiseco pistons built to improper spec...
-Phil