I would argue the aerodynamics of a helmet depends greatly on the bike and the position of the rider. Most of us don’t ride with our chin on the tank seeking that last extra 2mph. A helmet shaped to work well in that position is not necessarily the best for sitting straight up behind a windshield. I have a Scorpion EXO-400 which is on the cheaper and heavier side but I also have a tall windshield. I only get minimal buffeting on my helmet and after a full day riding I have never experienced neck pain. Sort butt and cramped legs yes; sore neck, no.
Helmet fit is the most important thing to look for, more important than price or anything else. Good luck finding a well trained salesperson helping you determine the best fit. I think the onus is on you to try on as many as you can to find what works for you. The simple maxim of the tighter the better works well. I think I lucked out on the EXO-400 as the fit is so snug I have been unable to find a pair of sunglasses I can get on, yet I do not feel any pressure points after hours of riding.
I think you make a good point about shell sizes but do you know where you can find out information about these? Even if a manufacturer was only to make one size, if that just happened to be the best size for your head then that could be better than the more expensive brand. Are there any articles that talk about how big an issue this is? Any recommendations from SNELL etc? It’s just something that is not much talked about.