Is a MBP capable of 50 MB/S over wifi?

What limits your usage is your local hardware - modem, router, other connected devices. Your computer is capable of Gigabit Ethernet and hi-speed Wi-Fi of around 50 MB/s. If you have a good router such as an Apple AEBS, then it should also support such speeds for Wi-Fi. Any other devices must support at least 802.11n to support that speed. However, any slower device you connect will cause the entire network to slow down to the speed of the slowest device on the network.

The router has to "downshift" and sniff for devices at the slower speed from time-to-time. While it is doing that, Routers generally cannot also listen or talk at the highest speed.

The router must slow down the data flow for any slow device on the network. This means all other faster devices will only get data at the slower rate. That's why you must upgrade all your network devices to handle the maximum speeds of your router. The router in turn must be able to handle the speed of the modem, and the modem must be able to handle the speed from the ISP. If you have a modem capable of handling 20 MB/s, then it makes no sense to have it connected to a 50 MB/s pipe.

A new AEBS is able to provide Gigabit Ethernet and hi-speed wireless in the neighborhood of 420 Mb/s or roughly 50 MB/s. Ethernet is capable of roughly 125 MB/s.

It's the device, not the type of Ethernet cable, that matters.

No, but the Wi-Fi system will. Wi-Fi and Ethernet are separate.

Comcast just increased my internet service speed to 50 megabytes per second downloads

Actually they didn't. Your speed is 50 megabits per second (Mb/s), which is 8 times slower than megabytes. Your network is also megabits.