I think you're being kind of generous. Not saying you're wrong. I believe your estimate could be accurate, but it depends on more than just switching out the helmet. Primarily, can you maintain that aero position through a majority of the 26 miles? Most of us age-groupers have a difficult time holding position for an hour plus, even in the best conditions. Start throwing in heat and exhaustion and our position goes to hell in a hurry.

I've gotten pretty strong on the bike in triathlon over the last few years. I'm age group competitive if only on the bike segment. But even on a relatively flat course like I did in Miami earlier this year, it's easy to drop your head when you start to get tired. Aero helmets get really un-aero really fast when out of position.

My personal take is just like the advice most triathletes get. The most aero gains is the position you can hold the longest. That would include the helmet. And if you can't hold the helmet the majority of the race, then a roadie is going to be every bit as good as an aero when you're looking at your pedals.