The only problem I see is the size of the wire required for 20amps #12 doesn't fit the 15 amp receptacles very well. You can't use the quick wire ( too big ) and it doesn't fit nicely around the screw on the receptacle. Meaning...if you can't secure it properly you can potentially have lose wires. its also difficult to get 3 conductors either BX/Romex/Lumex spliced with pig tails, marretts & the receptacle into your small box without pinching wires, You can run #12 from the panel to a junction box then drop down with #14 to the receptacle but that junction box has to be accessible and that will fix wires being too big to for the plugs and boxes.
