Is it possible to run os x 10.8 on a modern mac?
Dual booting is likely out, the older OS won't have the drivers for newer hardware.  What you could do is run the older OS in a virtual machine.  There's VMWare Fusion, Parallels, VirtualBox, and maybe more I'm missing.
I run virtual machines routinely and don't experience performance issues so long as there is enough RAM.  As I plan to run virtual machines I buy computers with plenty of RAM from the start.  After that the performance is limited like it was installed &quot;on the metal&quot;.  There's some quirks with virtual machines that may or may not be problematic and they generally have workarounds.  One issue that comes up is the guest OS is effectively behind a NAT internet sharing from the host, if this is a problem then it can be rectified with a USB network adapter.