That's not a small compromise,
a regular buck converter will not work correctly off of a solar panel. Please read what I posted earlier again:
You don't just need a DC/DC converter, you need an MPPT. Look up the I/V curve for a solar panel. It's non-linear...at either no load or short circuit they deliver 0 power. Maximum power occurs somewhere in the middle. Where, exactly, depends entirely on solar irradiance, temperature, etc. If you just stick a DC/DC converter on a panel's output, as soon as the motor draws more power than the panel can provide (cloud passes by, etc), the DC/DC converter will drop its input impedance to compensate for the voltage drop, and you'll fall off of the edge of the panel's I/V curve and get essentially zero power, indefinitely.
It's not just a matter of losing some efficiency, if you don't use SOME kind of power tracker intended for use with a solar panel, there's a good chance things will work ok for a while, and then just stop. It'll get stuck in an infinite reset cycle, maybe overheat itself, who knows. BEST case scenario is that doesn't happen, but you get SIGNIFICANTLY (read: half, third, etc) less power than the panel is capable of providing.
A regular DC/DC converter expects a CV (constant voltage) supply, a solar panel is a CC (constant current) supply. You can't stick a DC/DC converter that's designed for a CV supply on a CC supply and expect it to work correctly.