You are correct that the speed that claim are theoretical, but Broadcom claim to achieve 1.3Gbps wirelessly.
"The top tier BCM4360 supports the PCIe interface and uses a three-stream implementation, reaching as high as 1.3 Gbps of throughput. The BCM4352 and BCM43526 support PCIe and USB interfaces, respectively, and use a two-stream 802.11ac implementation that can reach up to 867Mbps. Lastly, the BCM43516 chip also supports USB interfaces, but uses a single-stream radio that can reach speeds of up to 433Mbps. By comparison, even multiple-stream 802.11n devices don't typically surpass 300Mbps."http://www.techspot.com/news/46909-broadcom-introduces-first-gigabit-speed-80211ac-chips.html
But, like with all technology, to achieve the speed, you have to have match equipment.