That actually isn't true. Which part?
Let's see. The more you cool, the more your pressure drops.
Stock TMIC, better cooling, higher pressure drop.
Aftermarket TMIC, poor cooling, lower pressure drop.
The reason for this is because the channels in the aftermarket TMIC are larger. That is great for lots of flow, but poor for heat transfer. The large channels mean less pressure drop, and poor cooling due to those channels helps more.
Honestly, the problem is the direction and design of the WRX/LGT TMIC more than anything. The shorter the passage, the better the pressure drop. So with more channels that are shorter, you can make smaller channels with better heat transfer and you get a better overall TMIC. (Lower temps and less pressure drop) But, the turbo layout doesn't allow for that...
The plastic is MUCH better than aluminum for flow and heat if it is reinforced...