Energy efficiency is a critical challenge for next-generation mobile networks. Especially as traffic demand grows, the energy per bit must decrease significantly. One promising solution is the Gearbox-PHY, which adaptively switches between modulation schemes and tailored radio front ends to maximize energy efficiency while delivering required data rates. In this regard, high spectral efficiency needs can be addressed with standard quadrature amplitude modulation, while low-power alternatives like impulse radio are employed for lower data rate scenarios, significantly reducing front end power consumption. While we considered the energy optimization for such a Gearbox-PHY in prior work, the specific focus of this paper is the consideration of the trade-off between oscillator power consumption and its phase noise. Using literature-based models and measurements for hardware power consumption, we demonstrate that embracing hardware impairments can lead to substantial energy savings of up to three orders of magnitude.