Well, here is what Cisco says on this:
"Because the switch stack behaves like a single unit, upgrades apply universally to all members of the stack at once. This means that if an original stack contains both EMI and SMI functions on the various switches, the first time a Cisco IOS Software upgrade is applied, all units in the stack will take on the characteristic of the image applied. While this makes it much more efficient to add functionality to the stack, it is important to make sure all applicable upgrade licenses have been purchased before allowing units to be upgraded from SMI to EMI functions. Otherwise, those units will be in violation of Cisco IOS Software policy."
So yes, providing you but the license (to keep things "legal") this will work just fine.
The StackWise technology thingie is a 32Gbps RPR type ring.
Here's ciscos quote on speed, etc.:
"To efficiently load balance the traffic, packets are allocated between two logical counter-rotating paths. Each counter-rotating path supports 16-Gbps of traffic for a total of 32 Gbps. The egress queues calculate path usage to ensure that the traffic load is equally partitioned."
Warren Kumari.
Snr Network Engineer
Google
