A PCI slot doesn't max out at 300Mbps, the network adapter does. They're completely different things.
The speed of a network adapter is linked to the wireless standard it uses (802.11a/b/g/n/ac) and whether or not MIMO or simultaneous dual-band is supported. For example, a basic 802.11n adapter has a maximum speed of 300Mbps, but if that adapter (and devices integral to the wireless network) support MIMO or simultaneous dual-band, then the speed can double to 600Mbps.