You guys make me feel so old! NO WAY is the heat crossover primarily there to heat the automatic choke. There were methods to heat the carb area way before the first automatic choke. It was done to promote fuel vaporization and to prevent carb freezeup. Look at a six cylinder from the 30s. The intake and exhaust come together below the carb.

High performance is a different story, the cooler the intake manifold the more power. So heat risers are blocked off and manifolds have the valve or whatever you call it removed. I ran a chevy for years in North Dakota with the exhaust damper removed and it worked winter or summer, so some heating must take place without the damper. I used a carb with electric choke and later with no choke at all.
The LA crossover tends to carbon up, that is true. It probably makes a stock automatic choke come off later, but I can't say.

R.