Well, it took me a while, but I have to say!
I just got one of the aforesaid Mid 2007 Mac minis, put Lion onto it (I bought a copy of it and had it made into a USB thumb drive as well as dual-booting it with other Mac OS X versions, and I can say that if Lion is this good (Snow Leopard took 5 vintage compatibility features away compared to just 1 for original Leopard) Mountain Lion, even though it's hardware investment will be far greater, can only bring better things.
(I just hate that I will have to buy all my applications all over again once non-App Store applications are blocked, but when you compare Gatekeeper at maximum security to Windows 8 or the optimized Windows 8 that is tentatively called 9, I have no desire to turn my back on a product that just works.
(Especially how when with Lion you can sit there and see a 2007-vintage Mac mini work like a new one, if with less memory headroom and a much slower chip.
It may have taken going on 4 years to confirm the OP's statements, but when I added the mini along with a 2008 model iMac 24-inch and Mid 2007 MacBook 2.16GHz, I was succumbed.
I am as much in awe as the OP was, just substituting an iMac 2.8GHz Core 2 Duo for the Apple TV!