I had similar concerns (early on) about the rear sight being in front of the RMR but gave it a try on the P09.
After shooting the P09 and P07 (I think both the slide Raining Brass shared are mine - Thanks very much, I LIKE'EM, ALOT!) with the rear sight in front I find:
1) I don't notice it unless I want to use the back up iron sights (that would most likely be true for either location)
2) the amount of the lens the rear sight obscures is the same, whether in front or behind the RMR
3) all the powder residue I wipe off the rear sight is powder residue I don't have on the RMR lens.
4) I feel (no proof) that the rear sight location, if in front of the RMR, helps protect the RMR from a badly deflected piece of brass
5) I find that the rear sight gives me a place to help gain traction on the slide when pulling it to the rear without using the RMR.
6) I bought a $10 lens pen at Walmart (well, two actually) to help clean the lens of the RMR
I'm using the dual illuminated RMR, too. One green and one amber (the amber is brighter, easier to see under less than favorable conditions).
I recently bought a Leupold Delta Point 3 and back up sight for the next pistol I have a red dot installed on. Just waiting to come across that P01 with the trigger/safety set up like my P07/P09.
Good luck with yours. I'm usually pretty tight with my money and took me a long time to talk myself into doing that to a handgun (and spending the money) but I can tell you that it's been worth it for me.
Oh, picture of the whole P09.