Not an ace, most of my work is simply CK work and more recently basic colour correction and compositing.
But, I do run some pretty large 1080p jobs 2-3 times a year for my kids productions which run about 2 hours of footage from 2-3 cameras and multiple audio sources, lots of transitions etc and it's here that having the grunt is sanity saving.
Regarding your 7950 vs 5780 question, I have both and will run Bruce's benchmark on both over the weekend and we'll see what the result is. I am pretty confident that the 5K benchmark doesn't really hit CPU OR disk that hard (it writes less than 20MB/sec, more than easily managed by a HDD), so this should be a great test.
I tried this on my MacMini 2012 with 16Gigs of ram, an SSD and TB attached hd and the integrated AMD 6630 graphics card, a BTO option and FCPx just sat there, nothing happened - spinning beach ball! On re reading the first post I see the words large amount of GPU Ram and the kicker is that I only have 256Mb! You may ask who the hell edits with just 256Mb of Graphics Ram on a $800 machine but I have been for the past 1.5 years!
Now here is the interesting thing, fired up my MBA, mid 2012 and it worked just fine:
Mac Book Air - Mid 2012
2 Ghz i7 - 8 gigs of Ram
Graphics Intel HD Graphics 4000 - 1024 MB
4mins and 30sec = 270sec
The secret here is to max out your ram on a the Air and this then assigns more ram to graphics card. My buddy got a newer Mini with the 4000HD card and he is seeing 1.5Gb or so GPU ram I think.
But looking at the limited RAM you may think it's time to dump the mini but the Mini, even with it's limited GPU WAS FASTER on some tests I did (std HD 1080p - looking at my buddies machine with HD4000 and 1.5 Gb or GPU RAM ) when I wasn't maxing out the GPU ram so I'm not sure what we can say from this test ???? Like I said, my mini has been able to open and edit a big projects with 1200 clips and I run a second monitor! Poor little machine! I also make sure FCPx is the only program running but sometime have a few others open.
I think we need a series of tests, this one is great but also a test to convert to proxy and other stuff. 4k is great but I wonder if it won't perhaps be like Super Audio - you know, the standard that superseded the CD ....