Iain McDonald announced that Hyper-V R2 will support up to 64 physical logical processors. Using 8 virtual processors per logical processor that gives us 512 virtual processors per host!
Processor Compatibility Mode is used in Hyper-V R2 clusters where you have different generations of processors from the same family. This masks CPU functionality that might be in newer node processors but not in older node processors. You still must stay either all Intel or all AMD.