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Hmm, unsure I could find you a specific reference, but I've used/use original 3750 series, 3750-E series and 3750-X series, and I can confirm that somehow data flows between stack members when the units are stacked and the stack ring is operating correctly. In fact, in this configuration, connecting a pair of Ethernet ports between stack members, within the same VLAN, will create a L2 loop.
Not documented (that I've found) is how StackWise selects which stack ring port (of the two) to use, i.e. whether it keeps all packets of the same flow on the same ring port, or alternates them. Also not documented (that I've found) is whether balancing is Etherchannel like (i.e. hashed), round-robin or some form of dynamic load balancing.
What is described for StackWisePlus, it only places unicast traffic on the ring when it must and the destination switch removes it. Original StackWise floods all packets onto the ring and the source switch removes it. (The StackWisePlus would better utilize ring bandwidth.)
BTW, StackWise is 8 Gbps (duplex) per port. StackWisePlus is 16 Gbps (duplex) per port. (Also note, the new 3850s might be 120 Gbps [duplex] per port.)
