First, most duplex outlets are rated at 15 Amps, not 20. Read the specs and weep. So, you should never pull a full 20 Amps. from a typical outlet. They do make 20 Amp. outlets, but they are rarely used as they cost 5 or 10 times as much. Volume pricing.
Even the 15 Amp. outlets are probably good to pass 20 Amps. through to the second set of terminals (screws), but I wouldn't guarantee it. I personally feel it is better practice to attach pigtails to the outlet and wire nut them to the feeds coming into and out of the box. But this may require a bigger box to hold all of this. And in your case, you would have four wires in each wire nut.
Wire Mold and other manufacturers make box extensions to help in situations like this. They stick out from the wall about 3/4" or so and add volume to the original box. The Wire Mold versions also have knockouts that are sized and shaped for the Wire Mold surface conduit. This is the professional way of adding a surface run to another outlet box.
I don't think you need to add a junction box up above unless it makes the run to the new box easier.
Make it fit.
You can't win and there is a penalty for trying!