imac performance and overclocking questions..

turbo boost is just a short burst from the cpu when it detects a heavy load. it changes nothing in the memory or clocking. It is not for "overclocking" your system to make it run faster.

 

Just wait...

Just wait for the next generation Mac Pro to come out if you are so concerned about power.

 

The PXW-Z100 writes about 60 megabytes per sec at 4k, using H.264. It would appear your main concern is I/O, however H.264 is also computationally intensive to edit due to inter-frame compression.

Current benchmarks indicate the new 8-core Mac Pro is roughly 1.8x the performance of a 2013 iMac 4-core 3.5Ghz i7. So it has greater performance but mainly due to more cores.

Editing 4k video is a cutting edge activity, but it's conceivable a 2013 3.5Ghz i7 imac might be able to handle that if equipped with a RAID disk array.

Note GPU performance typically doesn't help rendering, as this is a CPU-bound activity. However depending on the editing suite, GPU performance can help effects (blur, sharpen, color correction, etc).

If rendering is your main concern, the latest Intel CPUs have improved Quick Sync instructions. To date these have not been well supported by editing suites, but it can potentially improve rendering performance by several hundred % for the supported codecs: http://en.wikipedia.org/wiki/Intel_Quick_Sync_Video

You need to talk to someone editing 4k video on an iMac or current Mac Pro using the software you intend to use. That would help guide projections for the upcoming Mac Pro vs iMac. Even on the same hardware, algorithm and codec efficiency can vary greatly between different editing suites, so you need to know the exact codec and software they're using.

You cannot manually overclock an iMac but the intel CPU "turbo boost" is automatically overclocking. I do 1080p Premiere Pro CS6 video editing using a 4Ghz i7-875k Windows machine and RAID 0 disk array. My wife has a 2012 iMac 3.4Ghz i7 iMac with 3TB Fusion Drive. In general her computer is faster than mine, at least for the benchmarks I've run on both. Despite the lower nominal clock speed, her CPU is two generations newer, so clock-for-clock performance is better, plus auto-overclocking is effective, as is Fusion Drive. A 2013 3.5Ghz i7 iMac is probably 5-10% faster than that.