in a word ... nope.
Plenty of info on here re: sub-maximal training give it a
search and see what ya come up with.
Actually having said that your question was will it
slow your gains ... it might do because after it all it is a form of cardio (and a good form at that) and any cardio done excessively will slow
mass gain (because it is using calories that would otherwise be put toward
building muscle).
I myself train taekwondo every day for 2 or 3 hours and am thus far having no real troubles putting on weight in the gym but my trick where that is concerned is to eat a buttload of carbs every day ... works for me.
My initial nope was more directed at the idea of boxing straining your muscles to the point it will put them in a constant state of load fatigue (as when using weights as you described) ... I seriously doubt it would.
that's what I rekon.