Power has a pretty specific definition:
Argh. Sorry. I mean force. Got my English physics terms confused again. The German for "power (physics)" retranslates as "performance"...
Keeping the arms relatively long and moving the major muscular attachments distally to enhance their moment arms make much more sense if you are using the arm to break/dig stuff, as in fossorial mammals, but that doesn't seem to be what they are doing.
Moving the muscle attachment sites distally _is not possible_. Vertebrate development is constrained; it's not possible to end up with a triangular arm. Even in animals with a propatagium, the biceps inserts near the elbow; to get the insertion site of the biceps closer to the wrist, all you can do is shorten the forearm so the wrist gets closer to the elbow. This is seen in tyrannosaurids and alvarezsaurids. To a much smaller degree, it's also seen in Neandertalers as opposed to average *H. s. sapiens*, and in the latter as opposed to me.