The fact there is no codes stored complicates the diagnosis. If the Drive-by-Wire is hiccuping, you would get a light on the dash indicating a Drive-by-Wire malfunction. The Venturi, butterfly and lightning bolt symbol. When I have had the drive-by-wire hiccup, it had the same symptoms you're describing but the warning light symbol flashed and remained lit on the dash. The only time it has happen to me was when I started the motor in extreme cold, like single digits fariheight, then tried to apply WOT or close to it before the motor had warmed up. Shutting off and restarting the motor cleared the problem.
LIMP mode for the transmission, I've never experienced it with drive-by-wire, but usually when the trans goes into limp mode, it locks into a lower gear and won't shift out. So, you're speed limited, but the engine functions normally, and pushing the throttle will cause the engine to respond and rpm increase.
You seem to describe an engine problem, when pushing the throttle pedal, doesn't produce additional power, the engine rpm won't climb, the engine is bogging down. That doesn't sound like the trans going into limp mode to me.