"you won't necessarily feel a difference in power -- it just changes where the shift points are."

Absolutely.

The only time I've used mine was while towing. The change in shift points helped to downshift on hills - made a big difference. Soooo....

Try a hard acceleration from stop all the way up to cruising speed with the switch off. Try it again with the switch on. Pay attention to the shift points & look for any change.

Other than that, I'll second the motion to do a computer reset first since that's easiest. Just disconnect the battery for 30 minutes - reconnect & try 'er out.

Second, check all fuses (next easiest)

Third, start tracing the wires from the switch to the computer looking for broken or frayed wires or loose connectors.

After that, I haven't a clue.

Be sure to post whatever you find.