I have 27mm ext 19mm internal Fast Forward wheels with 23mm GP4000 SII. I upgraded the wheelset on a 2013 Cervelo S2, which has very low vertical clearance, so its the biggest tire I could fit. The 23 balloons out to 25mm, seems like a pretty good interface with the rim from an aero perspective.
I discovered through trial and error that unless about 3mm clearance is needed between the tire and frame otherwise a wet ride will turn your tire into a sanding wheel and take the paint off the bottom of your fork. No more 25’s on this bike.