Besides screwing with my articulations and my intonation, which is something mutes do anyway, I've noticed that the simple weight of the mute added to my horn must be too much for me to support entirely with my left hand. I've begun subconsciously shifting weight to my right hand -- the slide hand -- which does all kinds of crappy things to one's tone. Most noticeably, it's made my slide movement slower, resulting in glisses between the notes. However, if I consciously attend to moving the weight of the horn back to my left hand, my arm gets very tired after about twenty minutes.
And so, deriving some inspiration from Mark Twain, I decided to let my neighbors know the true meaning of the opening line,
If it please your neighbor to break the sacred calm of night with the snorting of an unholy trombone, it is your duty to put up with his wretched music and your privilege to pity him for the unhappy instinct that moves him to delight in such discordant sounds.I took the mute out. I still practice at reasonable hours of the day, and I try to sound nice, but the whole exercise has really helped. My high range opened up, my weight and balance issues vanished, and what's more, I've developed a whole new sense of confidence. I make these people listen to me flub all those F5s I've been working for, so maybe they won't mind listening to me drill the syncopated lines and running sixteenths in Eric Whitacre's Equus until I (or they) puke.
The only response I've gotten so far is my upstairs neighbors turning up their television. It's a fine trade-off for improved performance and confidence!