A quick search of WikiPedia says that a mini-PCI is 32bit @ 33MHz with a maximum bitrate of 1.067Gbps. Not exactly enough to deal with 3x3 MIMO 802.11ac for example, but maybe enough for an 802.11n link?
Even mini-PCIe with its one lane "should" do 5Gbps (PCIe rev 2). I suspect a proportionally meatier CPU would be required to service the interrupts, etc. though.