My mistake, there is no early 2008, I was looking at the early 2009. Of course my late 2008 MacBook Air here (which was already underpowered for the time) is still supported by Yosemite. The reason the early 2008 MBA and mid 2007 Mac mini stopped at 10.7.5 is because of 32-bit EFI. Another thing to note is that while '06 Mac Pros stopped at 10.7.5 for the same reason, they can run Yosemite quite easily with a bootloader.
It is actually possible to do a firmware update on 2006/7 Mini to a 2,1 and if using a Core2Duo you can then install Yosemiteusing same boot.efi as Mac Pro 1,1. The only issue is GPU. There are extensions to partially enable the GMA950, but you don't get full acceleration.
I am using just such a setup to test WiFi AC/BT 4.0 cards. And yes, with one installed you can use Handoff/continuity with no additional hacks.
Had Apple chosen to include GPU drivers they could still be supporting these with ease.
As far as 2014 that is rated at 1.4 Ghz, if you use Intel's CPU tool in Windows, the CPU is always at 2.0 Ghz and uses 13 Watts. Running Far Cry 4 at 4K 60Hz with effects maxed it never breaks a sweat. Most of the work is done by eGPU, the CPU doesn't seem to do much.