I am switching my P-Series to tubeless with GP5000 tires. To improve comfort, I would like to go with a 28mm rear tire. The front tire will be 25mm for improved aerodynamics.

I have lightbicycle ar65 wheels which measure 28.5mm wide externally. A 28mm rear tire would then violate the “rule of 105”. A 28mm tire also has more frontal area than a 25mm. But the rear tire is shielded by the seat tube, so I am wondering if these supposed aero penalties are not really that significant since the tire is shielded. Does anyone have any experience with this? Is there a minimal watts of drag difference from a 25mm to 28mm rear tire?