regarding VP9, you can force youtube on chrome to use h264 (and you would want to do that if you have a GPU with fixed function acceleration and slow CPU, but h264 on chrome is a lot more demanding than running it on a program like MPCHC) by using a simple extension called h264ify, since only the GTX 950/960 (GM206) with forcerware 361 I think actually supports VP9, if you don't have a GPU capable of H264 acceleration it might be better to use VP9 anyway (it got a little lower CPU usage on the limited test I've done with a t4400 laptop with software only)
also, all the Radeons that I have (4670, 5570, 5850) can't handle youtube 1080P 60FPS with h264 acceleration (720p60 works well, same for 1080p30), so it might be worth using VP9 (with the CPU) for those.
VP9 4K60FPS on youtube can be brutal my 3Ghz sandy Bridge i5 can't handle it well enough, I suppose an i7 2600 would handle it well enough.
and yes, for the older hardware it's probably best to use older software with better support, trying to decode DVD with MPCHC gave me some not so great result with slower CPUs,
I thought CoreAVC would have a larger advantage, I remember it doing some miracles back in 2007-2008 with my A64 X2 trying to run some demanding 1080P samples, but once I started using GPU acceleration (from a 8600GT) it was no longer needed, also h264 when it works is pretty amazing, I was running XBMC and MPCHC at some point with a p3 650 and the 8400GS PCI and it could run some HD h264 videos quite fine with low CPU usage due to the GPU (but it was totally useless for web browser videos, even with fast CPU, I think because of the PCI bandwidth)