As in most hybrid project you will end up rather with a combination of all disadvantages than with good points.
As a matter of fact the C-123 was using the Jet to increase Take-off and climb perfs.
I think, as you suggested, the prop will generate more drag than thrust beyond a given speed... which will invariably increase your fuel consumption.
The Aircraft used in Conair (what a crappy movie !) was a C-123 Provider. You could have mentionned "Air America" as another famous operator . Who was flying Provider from during the 60's in Asia, til the mid 80's in South America, and probably in some other places I don't want to know about ....
Amazingly before being fitted with 2 turbojet, the C-123 was a classic piston twin . Therefore we can consider it was a succesfull hybrid (it's rarely the case in technology history )
What people don't know about the C-123 ( I know , I disgress ) is that, before being fitted with piston engine, it was fitted with ...nothing. Originally, it was designed as a glider !
The glider had so good flying characteristics that it was decided to turn it into an aircraft .