Did anyone mention forced transition? What is the line between thick and thin? RC Pattern... 10% is thin... FAI F3B... 7% is thin... What is usuially "GOOD" is a rounded leading edge... and if the Rn is very low, then TURBULATE it. Use Zig Zag tape if you must, but even pin striping tape is usually enuff. Full size SAILPLANES are KING of EFFICIENCY... they typically run in the 15 to 17% thicknesses... although they are tending thinner as Carbon Fiber comes down in price. So the short answer is don't make it any thicker then it NEEDS to be! AERO is a science of compromises... what set of compromises result in the best solution... that question will lead you to the correct answer.
My own personal observation of how different thickness airfoils behave is this: Thick sections will stall at a LOWER Angle Of Attack then a thin section, but the thin section will stall more abruptly. I suspect this is due to the fact that air does not like flowing around curves, especially in a flow field where the pressure is increasing, ie. top of the wing behind the thick point. (Adverse Pressure Gradient) The thick section typically has MORE curvature in this case then the thin section. When the thin section does finally depart, it does however with less indication and more suddenness... (This is admittedly a subjective opinion/observation)