I’ve got to think that the airflow around the back of your saddle is completely disrupted by the turbulent airflow off the rider, which means that the aerodynamics of your saddlebag have zero effect on your total drag. I mean, that’s the reason why triathletes put their water bottles behind their saddles, right?
I am a big fan of speedsleeve.
While I agree that the drag from a seat bag is probably minimal (at least in terms of different bags), there is a difference between “under” the saddle and “behind” the saddle…not to mention a significant size difference between water bottles and a seat bag.
Your most aero option is probably a BTS mount and using a storage keg in place of a traditional water bottle.
However, there are some aesthetic / roadie rule issues that come along with that choice.
I tried the bottle behind the seat thing. The issue is that even just normal cracks in the road like to elect the bottle / keg. And that’s with using cages that many said were very secure. So the only way I would trust that kind of setup is with a strap that makes it impossible for bumps to eject the keg.
With stuff in your jersey, those items could hurt your back or organs if you get hit or fall. I have read too many horror stories of spinal damage. For that reason if I carry metal items I will only put them in the side pockets. I prefer to have nothing metal but my phone in my pockets.
I am debating between the Cyckit Aeroclam and the Lezyne Power Pod QR or Silca Seat Capsule Premio.