Idle and progression circuits are connected as the idle jet is responsible for the fuel supply to both circuits. Therefore the size of idle jets affects the transition circuit as well.
Once the butterfly is fully open the motor is running on the main jet and air corrector jets, however when cruising at constant speed, the throttle valve is usually opened only partially, so the progression circuit is still engaged. That circuit is important for cruising performance as when transitioning from idle towards WOT. If the idle jet is too small or too large, it will cause the carburetor to go lean or too rich and the engine will stumble.
DCOE carburetors came in a variety of types over the years. Variations include number of progression holes, pattern of progression holes, different floats, etc. The 152 type is the current commonly available new stock. It has many desirable features and excellent resistance to wear. One of good features is increased number of progression holes for smooth transition from idle.
The fuel level is also important factor that affect the progression circuit. If memory serves me, you installed top cover from your old carburetor. The fuel level setting is NOT the same for DCOE 9 and DCOE 152, so that probably affected the engine negatively. I do not have exact information for DCOE 9 but for DCOE 152, fuel level should be 27 mm from the bottom.
Also, I have always advocated against predetermined number of turns when setting up the idle mix screw. Idle mix screws came in several versions during the years. Later model DCOE Webers are fitted with narrow tapered screws that are more accurate and require more turns to flow the same amount as their predecessors. Therefore, the same number of turns is likely not valid for DCOE 9 and DCOE 152.
Proper adjustment procedure is (Redline instruction):
a. Start the engine, it will run slow and like a tractor. As long as it will stay running, the idle speed is not important at this point.
b. First, turn in the mixture screw until the engine runs worse, then back out the screw ¼ turn at a time. The engine should start to smooth out. Continue to back the screw out ¼ turn at a time until the screw does nothing or runs worse. Then turn it back in to the point where it ran best. You want to tune the engine by sound. Adjust each mixture screw to the best, fastest and smoothest running point. Do this procedure with each mixture screw.
c. Now you may adjust the Idle Speed Screw. It should be sensitive and only require ¼ turn in or out to achieve the idle speed you like.
PLEASE forget about predetermined number of mix screw turns and adjust them to the point when the engine idle the smoothest. Changing the idle jets will require re-adjustment of mix screw.