#4 wire rate means packets are sent at the maximum capacity of the wire. It's measured in packets per second, or PPS. For Ethernet, 10 Mbps wire rate, using minimum size packets (also w/o a VLAN header), is 14,880.0952 PPS. For other Ethernet bandwidth, scale up or down based on bandwidth. (NB: as packet size increases, PPS decreases [much] to maintain wire rate. Also as packet size increases the percentage of bandwidth devoted to overhead decreases [much] too.)
For your other questions, related to stacking, it gets a complicated parsing out real world performance/capacity vs. what the vendor's "sales" literature states.
