He's always been portrayed as being an average Imp.
Feat wise, like galan said the most powerful from that dimension, but only in the 3D.
Its been stated that, 5D imps percieve the 3rd dimension as a sheet of paper, they can do what ever they want on that piece of paper, erase what they what, or just rip it to pieces and dispose it.
So say for arguments sake franklin richards was transported to the 5th dimension, he would be powerless.
The source of imps power is also complicated this is how many believe it is........
Mxy's power comes from VERY advanced imp tech, but because it is pretty much impossible to understand and thus it is percieved by the DCU as magic, it used to be just magic, then it got retconned to what i just said.
So if any other 5D Imp has the same tech or maybe even more advanced, they would be capable of what mxy is capable of and more