Qatsius check the Pirelli box, which states the internal rim width from which the nominal tire size is derived. I believe both 26mm and the 28mm sizing comes from 19mm IW rims, in which case yes, you should expect them to measure a bit (1mm seems reasonable) larger. FWIW, the 30mm P-Zed Race TLR RS are measured on 21mm IW rims, so checking the box for details is the way to go.

I can't venture a guess as to how the aero will play out comparatively, so I'd make the decision based on how I want the feel to be. I do like the razor crisp feel of 25mm Michelin Power Cup on 21mm IW Lun Hyper wheels at 90psi, but Pirelli P Zero Race 28s on 23mm IW Fulcrum Racing Red 900s (a shallow, Al rim) at 80psi did not seem to be giving up anything to the Power Cups in terms of sporty feel, despite being more forgiving of road surface and more comfortable. Ultimately-- i.e. at max efforts-- the deep section Hyper with the 25s were faster for the aero effect, I think, but solution was to get deeper, wider wheels to accomodate wider 30mm rubber! So that's where I'm coming from: I'd do 28s on the ARs.