Perhaps you are trying the wrong strategy.
Given your needs as a developer, why not consider purchasing two inexpensive Windows PCs, one to run Windows 7, and one to run Windows 8, and then hooking up those two machines and your Mac mini with a KVM switch so that you can use the one keyboard, mouse and monitor to control all three computers, which are all up and running simultaneously?
It seems to me that you would save a great deal of time because you would never have to reboot into another OS to test something. With only one monitor, keyboard and mouse, and the two other PCs on the floor, you would not take up any extra desk space either.
With a router nearby, you could connect all three computers with Ethernet or WiFi and set them up for file sharing through the different operating systems, or using various versions of VNC. Then you could easily move documents and files from the Mac to the Windows 7 or Windows 8 environments in real-time while all three are up and running.
You could purchase new desktop PCs with a 500GB hard drive, 4GB RAM, and a licensed copy of Windows 8 or Windows 7 for perhaps US $300 each.
Alternately, you could purchase only one $300 Windows computer and set that up to dual-boot into either Windows 7 or Windows 8. You would still use the KVM switch between your Mac mini and the one Windows PC, and that would cut down somewhat on the amount of time you lost rebooting between different OS. But I think purchasing two cheap Windows boxes alongside your Mac mini would be the way to go.