I spoke to NGK USA a few weeks ago about these. Now, because they don't offer them (yet, or ever - as the guy tried to lead me to believe) take this info with a grain of salt...
Supposedly the RX series is designed for countries with poorer quality gas than that sold across North America and Europe. Its also cheaper to manufacture than Iridium and offers most of the same benefits with approx. 50-75% sparkplug lifespan when compared to regular Iridium plugs.
So if you live in areas where the gas quality is not that great and you can get a set that matches your car - great. Otherwise NGK USA advised me against them.
Reading up a bit further on the difference between OEM Iridium, Iridium IX and Laser Iridium (stock on the 3.6R) - the best plug with the longest duty cycle is the Laser Iridium series. IXs are a marketing gimmick, the 'cheapest' of the three. OEM Iridium come in second, Laser Iridium first. They can go as long as 100K miles before needed replacement, although Subaru still recommends new plugs every 52 (new info) and 60K (car manual).
Now, if you're going to swap the plugs every 30K; it probably makes little to no difference which you go with. All will be better than non-Ruthenium or non-Iridium plugs or any of the other brands. But if you want to go 50-60-100K, Laser Iridium with the right heat rate and thread config.
If you're Stage 1+, you can consider going one heat rate higher (colder). For Stage 2+, I'd recommend it and consider a re-tune around the time you swap them out.