These things aren't "seat belts" or "harnesses", they are a SAFETY harness.
1. A fifth point sub belt does not make the harness uncomfortable.
2. Each component (lap, shoulder, and sub) plays an important role in maintaining your position in the seat during aerobatics, turbulence, impact, or maintaining the position of the harness on your torso.
Install the five point. You don't always have to use the sub belt if you feel it is uncomfortable, but there are times you will definitely want it (and perhaps unfortunately, times you will wish you had used all of the components of the safety harness).