I use Multibridge Pro for R3D editing and grading and used to do a lot of P2 stuff, too.
Multibridge Pro works with R3D footage well, the "native" clips from Log and Transfer at 2k displays on 1080 monitor as 16:9, so the full with displayed at overscan, letterboxed at ratio of 16:8, so black bars top and the bottom. 23.976 R3D native clip (actually wrapped QT) plays back pretty well. What you can do is to select Blackmagic 8-bit 1080 YUV output instead of 10-bit so FCP has easier time playing the native clip in realtime.
Then you can send the timeline to Color for grading with native R3D features. Again Multibridge Pro works well in Color. If your output will be for film or 444, then choose to display at Blackmagic RGB output choices, if video then choose YUV choices (choices mean 720, 1080 with whatever frame rate you're working on).
If you are working with 10-bit or 8-bit uncompressed, then you have no problem going back to FCP by "send to FCP". What you get back in FCP will be pretty much indentical in gamma and color. However, at this point, this combination lacks 10-bit 444 output. I pointed this out in another post, but I got no reply from Blackmagicdesign.
At this point, I render as DPX, manually conforming the rendered output in FCP (using GlueTools with my special white point and black point values) then convert them to BMD444 uncompressed format, then reconform again with the original timeline. There should be an utility come out from Glue Tools to conform DPX rendered clips to FCP in the future.