Yes, you might not like it downshifting like that when normally driving. If you have it automatically downshift to 2nd at 40mph then you really can't have it automatically upshift to 3rd until above 40mph. So, you could find you're driving around with the revs too high.

You'll also probably find the 4L60e doesn't give a very good feel when forcing downshifts. There is no rev-matching so it would just kind of jerk into the lower gear.

You don't need the TCC locked up to get engine braking either.

The performance switch is a momentary pushbutton that grounds a pin. There should also be a PCM output that could be used to light an indicator.