To answer my original question, I found that the large hole in the VA-149-320 throttle/mixture bracket is indeed correctly sized for the MA-4SPA carburetor, which has a smaller throat bore than the MA4-5 carb and the sump opening on my engine. This allowed me to use the tapered sump insert that Titan machined for me, similar to the Lycoming Service Bulletin, and capture the insert with the bracket as Titan's engineers instructed.
I've only made one test flight since installing the insert and the new carb that Titan sent (which has a bunch of tiny holes drilled in the main jet to better atomize fuel), but results so far are extremely encouraging. The engine started much more quickly, all 4 CHTs in cruise fell in a 20 degF window, and EGTs were much closer as well. Further flights at varying altitudes and mixture settings will hopefully confirm this issue is resolved.
For anyone experiencing a massive CHT/EGT imbalance between the front 2 cylinders and the rears on their carbureted engines, I would encourage you to remove your carb and measure the diameter of the carb throat and the sump throat. If the sump throat is significantly larger, this could very well be the cause of your mixture distribution imbalance. Have a look at Lycoming SB258 - they figured it out in the 1950's. Not sure why we're still struggling with this issue almost 60 years later.