Pre-cats are inside the exhaust manifold. It isn’t that it is more efficient, there is no other reasonable way to pass federal (and especially California) emissions standards. Catalytic converters work really well, but not until they get hot. You have to realize that when an modern engine is running at full-power, the exhaust is at around the durability temp limit of the manifold, the pre-cats, and even the exhaust valves. The manifold is glowing red-hot (the reason for heat shields). In my career with many engines in the dyno cell, I have had 2 engines ruined by the cat substrate fail from heat cracking. Over the past couple of decades, newer engines have proven to have pre-cats more durable due to better materials and engine control systems.
The full-power exhaust temp for modern engines (running normally) are anywhere from 850 to 1050 degrees C. Up until recently, all gasoline engines run rich air/fuel ratios at very high loads to manage the peak exhaust temp. Running rich at full-power creates a lot of Hydrocarbon and CO emissions and hurts fuel economy, but does cool the exhaust. The emissions standards around the world allowed this but this is changing. The new standards ramping up between now and 2027 do not allow running rich and this is being handled by exhaust manifolds that are integrated into the aluminum cylinder heads (puts more heat into the cooling system), careful development of combustion and calibration, and better materials.
