I've done the mod to over 20 snow blowers, and every result has been positive. It doesn't bog down the machine from rubbing on the side, since they only rub for the first few revolutions until the paddles have worn in.
The paddles decrease the clearance between the impeller and the housing, making the machine more efficient. I've actually noticed it helps machines not bog down, because most of the bogging is the result of slush sticking to itself on the wall of the housing. This is why machines will bog down in heavy slush and clog up.
I do recommend that when you do this mod you do two other things. Replace the auger belt (if its a few years old) and wax the inside of the chute. These two things help tremendously on older and smaller machines.
If the machine bogs down it means the carb needs some adjusting, and you probably had problems with it bogging in heavy snow before you did the impeller mod. I like Tecumseh snow engines, but I still have nightmares about their carbs.
The carbs must be perfectly clean and tuned correctly for the machine to run properly. If you have to use the choke at all besides starting, then the engine is not getting enough fuel.
I would not hesitate to do this mod to any 2 stage snow blower, it simply makes the machine more efficient and is a night and day difference for the slushy stuff.