I think youre absolutely right in that any sight or optic of any kind, still has to be mounted and the mounting system can make a low profile optic unusable with some platforms... especially a shotgun.
I always liked the Bushnell I have, but not on my shotgun. It was difficult to use properly and made shooting with it seem like a chore.
As far as the profile similarities between the 590 and 930, I dont know for sure off the top of my head. I would think theyre very close. The surest way, and Im about to leave for work or I'd take a look real quick, is to get on OFM's site. Maybe open up tabs side by side. One for the 590 and the other for the 930. Get to the spec page for each and find the columns that show the DAC (drop at comb) and DAH (drop at heel) and also compare the LOP (length of pull) I cant guess what the difference in drop is at either point if there is a difference, but I bet theyre very close.