I'm using CS5 on Win7, patched to work with an nVidia GTX260 video card. If you can upgrade your Mac with a CUDA card I'd highly recommend it. I recently converted a number of interlaced GH1 1080p files to 720p, and Premiere CS5 displayed them just as smoothly before and after conversion.
Regardless of whether you can upgrade your video card, I'd definitely recommend deinterlacing your 1080p video files. There are many subtle things that can go awry with interlaced video and I would want to make sure it's correctly deinterlaced before attempting to edit the files. If you want to maintain interoperability with Final Cut, Cineform is a good option for both deinterlacing and transcoding to a portable format.
For 30p to 25p conversion, I'd suggest trying After Effects using its Motion Blur but not its Frame Blending option. I've used Motion Blur with very good results to convert 720p60 to 30p. Frame Blending is very likely to cause ghosting.
Regardless of whether you can upgrade your video card, I'd definitely recommend deinterlacing your 1080p video files. There are many subtle things that can go awry with interlaced video and I would want to make sure it's correctly deinterlaced before attempting to edit the files. If you want to maintain interoperability with Final Cut, Cineform is a good option for both deinterlacing and transcoding to a portable format.
For 30p to 25p conversion, I'd suggest trying After Effects using its Motion Blur but not its Frame Blending option. I've used Motion Blur with very good results to convert 720p60 to 30p. Frame Blending is very likely to cause ghosting.