I tested '12 R about a year ago then drove '13 R shortly before ordering an STI.
I have no idea why people would compare it to the STI. It's a bit better than the Limited WRX inside. First time I sat in the R was when I still had my spec.b, and I was totally unimpressed. With all the talk about interior quality it didn't (and still doesn't?) have seat position memory,for example. It's better than the interior of the 'Limited WRX', but not 'in the other league', as some people here tend to put it.
Performance-wise, I'm yet to see anything that would put it above limited WRX. So it really competes with Limited WRX. And the trade-off you get is 'a bit nicer interior' vs 'a bit crappier AWD'. And a way higher price.
And good luck flashing your R's. 'Mod-friendly' dealership means nothing, WV has them scan your ECUs during regular maintenance visits, and then quietly flags your car with TD1 flag, meaning you're F'ed if you need service and also F'ed when you're selling your car, since buyers of second-hand ones will be looking if that flag is set in the database.