Going past
patched Mountain Lion will lose graphics acceleration which makes the UI slow and glitchy.
Tried patched El Cap on my maxed-out 2,1
(upgraded CPU, max RAM, SSD) one time, and it was by far one of the single slowest computer experiences I've ever had. GPU performance was definitely the biggest issue, but overall the hardware couldn't handle it. Ended up going back to Snow Leopard after ~30 minutes of trying to use/troubleshoot it.
Mavericks may run slightly better on the CPU front (considering Merom was used in the 2007 MBP), but graphics will most likely still be painful.
Honestly, the GMA 950 is so weak (even by 2006 standards) that I wouldn't bother going past Snow Leopard with it. The 9400M was a desperately needed GPU upgrade for the Mac mini.