I just purchased the MotorGuide Tour Pro with the built-in US2 transducer. If I purchase a Helix and dumb it down to work with the US2, how much quality will I lose compared to the high frequency transducer that I could mount to the trolling motor foot? I would prefer to use the integrated transducer but I also don't want to be leaving too much quality on the table. I just can't visualize the quality loss. Can anyone speak to this? Or, better yet provide a comparison image?

It will be close in clarity and response. Enough so, that either will be fine. Now remember, the US2 doesn’t do anything other than 2d sonar and water temp. No down imaging and no Chirp sonar.

The standard for the newer units is High Wide CHIRP with a CHIRP rated Low-Q 2D piezo==true CHIRP.

The unit will CHIRP the 83/200 kHz US2 transducer if you don't turn CHIRP off===Cheap Chirp.
All Helix units starting with the second generation have the CHIRP setting by default.

Second generation Helix units CHIRPed the 83/200 kHz piezo at 75-95 kHz and 175-225 kHz.
The first generation Helix 12's also CHIRPed the 83/200 kHz piezo.

ANY CHIRP has better target separation than single frequency, HW is just better.