Earlier today I posted a guess about IM vs SC administration of B12. I decided to so some looking around and found this paper - ncbi.nlm.nih.gov/pmc/articl...
The results showed, for many of the drugs studied, there was little difference between the two routes.
Some of the individual results were interesting. For epinephrine (given for anaphylaxis - where speed is important) peak concentrations were reached faster with IM. But when they looked at several different sites for IM administration of epinephrine they found a huge difference between injections into the thigh and arm (the thigh was much, much better - presumed to be because of greater blood flow).
For gonadotrophin and hydrocortisone they found that body size can affect the maximum blood concentration for SC doses - a higher BMI meant lower blood levels.
Then I found this document - columbia.edu/itc/gsas/g9600... - which agrees with my reasoning but doesn't give any data - just theory.
In the future I think I'll be suggesting that there's probably not much difference and, considering how SC is less scary than IM, SC may be preferable for many.