The discovery of large terrestrial (~1 Earth mass (Me) to < 10 Me) extrasolar planets has prompted a debate as to the likelihood of plate tectonics on these planets. Canonical models assume classic basal heating scaling relationships remain valid for mixed heating systems with an appropriate internal temperature shift. Those scalings predict a rapid increase of convective velocities (Vrms) with increasing Rayleigh numbers (Ra) and non-dimensional heating rates (Q). To test this we conduct a sweep of 3-D numerical parameter space for mixed heating convection in isoviscous spherical shells. Our results show that while Vrms increases with increasing thermal Ra, it does so at a slower rate than predicted by bottom heated scaling relationships. Further, the Vrms decreases asymptotically with increasing Q. These results show that independent of specific rheologic assumptions (e.g., viscosity formulations, water effects, and lithosphere yielding), the differing energetics of mixed and basally heated systems can explain the discrepancy between different modeling groups. High-temperature, or young, planets with a large contribution from internal heating will operate in different scaling regimes compared to cooler-temperature, or older, planets that may have a larger relative contribution from basal heating. Thus, differences in predictions as to the likelihood of plate tectonics on exoplanets may well result from different models being more appropriate to different times in the thermal evolution of a terrestrial planet (as opposed to different rheologic assumptions as has often been assumed).