Sorry, but at this time there are no accessories that will boost the Wii U/GamePad signal strengths, so really the only other things in addition to what you have already done, is making sure that there is no other electronic device like a cordless phone, microwave oven aren't in use as they can interfere with the wireless signals.
But also, distance (not being over 25 feet) and there being a clear line-of-sight between the Wii U console and the GamePad, so that a wall, door or other electronic devices are not blocking or interrupting the signals going back and forth between the two.