This is probably a bit of a longshot, but I'm wondering if anyone has compared vmware fusion (windows xp, 7 or 8.1) in Snow Leopard to Yosemite on older hardware (core2duo, 8GB ram, ssd). I was planning on getting a new mini, but have decided to pass since it's crippled and non-upgradable. Going to keep running my 2008 macbook as long as possible. VM performance isn't the best and I'm wondering if it would get a boost by moving to a newer version of vmware fusion/Yosemite/Mavericks.
Chances are the newer versions of Fusion won't run much better, if at all on the older MBP. The performance hit you're seeing is all due to the older hardware you're running.
I think you'll be much better off with the "crippled" Mac Mini over your 2008 Mac mini. The newer versions will push your machine harder then what you're running now.