Hello,
I can not really tell you of a piece to use to increase speed, just a technique I use when I practice euphonium (9 years, odd that we both posted a thread mentioning euphonium at the same time)
Change the Rhythm
Get your metronome out , how else will you know you are getting any faster

set the metronome to as fast as you can play the exercise without messing up.
First Time Through: Play through an exercise at the normal rhythm and tempo.
Second Time Through: Swing it. Play the line as a dotted-eighth sixteenth (is there a way to show rhythms on this board?) COMPRESS THE RHYTHM!!!
Third Time Through: Do the exact opposite. Play the whole exercise as a sixteenth dotted-eighth. COMPRESS THE RHYTHM!!!
Fourth Time Through: Play the original Exercise making sure not to compress the rhythm at all ... its sooooo much easier now.
Increase the metronome marking and continue.
~
This exercise tricks you brain into thinking you can play the line faster. After all, you played every combination of notes twice as fast as you ever have before (yeah, you didn't do it all at the same time, but your brain doesn't need to know that, haha)
I have used this with scales on bass. I havent tried it with tapping.
~Divide and Conquer~
Brassed