Not QCX But in my U3s I have two DC-DC boards inside the official case alongside my U3s and relay board.
One runs at 5.5v and feeds the main board, which in turn feeds the gps, the other runs at 9v to feed the 3 pa transistors.
No problems whatsoever, u3s performs well with no drift on Wspr and QRSS is sent well. Obviously there is no receiver in the u3s so I can't comment on whether that would be affected. There is no filtering, I also run my FT817 from a DC-DC board but that is well filtered and double screened. Again, no problems.