Aerodynamics is complicated. Tires that are sized appropriately for their rims will get you bigger aero gains than just using a smaller front. Either way at recreational speeds any aero gains from tires won't be as much as the difference in rolling resistance between good and bad ones.

If you need 28s for comfort or grip, keep using them. If you want your bike to ride harsher, switch to 25s or even 23s and blow then up to 120 psi.