Internet Connection Speed SLOW!

Question

My Set-up...
2012 MacPro 3.33 GHz 6-Core Intel Xeon running Mavericks 10.9.5
Airport Extreme Base Station -  Card Type: AirPort Extreme  (0x14E4, 0x8E) - Supported PHY Modes: 802.11 a/b/g/n - Firmware Version: Broadcom BCM43xx 1.0 (5.106.98.100.22)
Modem - NEW Arris DOCSIS 3 CM820
Suddenlink High-Spped Cable (up to 50Mbps advertised in my area)
MackBook Pro 17" 2011 Model running Yosemite 10.10.2
Here is my problem. Last week Suddenlink increased the minimum bandwidth in my area to 50Mbps from 15Mmbps. They said I would need to upgrade to a DOCSIS 3 Modem to be able to take advantage of that speed increase however, so they sold me a new modem (Arris CM820) which I hooked up today.
I ran a Speed-Test at http://www.speakeasy.net/speedtest/ both with my old Motorola Modem just before I disconnected it, then ran the test again after I upgraded the modem. I am connected to BOTH my PowerMac 2012 and my MacBook Pro 2011 via the SAME WiFi Network Base Station (Airport Extreme). My older MacBook Pro immediately showed a HUGH Speed Gain when running the speed-test, but the MacPro "download speed" actually went DOWN??? What could cause this?
Mac Pro:
Before: 17.48 Mbps Download | 1.59 Mbps Upload
After: 3.32 Mbps Download | 5.37 Mpbs Upload
MacBook Pro:
Before: 17.32 Mbps Download | 1.52 Mbps Upload
After: 49.36 Mbps Download | 5.58 Mbps Upload

Answer 1

As far as your configuration goes, I do not see anything strongly out of whack. You may want to set IPv6 to link-local only (both here and on your Macs) since it is not yet used for most Home Networks (but may be used by Bonjour Service Discovery). It seems unusual that your Airport Admin port (50.27.113.156) is set to a "real" Internet address, and not just a 10.0.xxx.yyy "strictly-local" Address, such as Manually  10.0.1.201 (just beyond the range you are using for your Macs). Do not set anything to 10.0.1.254 -- that is reserved for Broadcast your local network.
The way it is set now, I can ping that Admin address from across the Internet. It is showing about a 75 msec answer time at this writing, and no lost packets.
In the 2.4GHz band, there are at most three clear channels at higher speeds (where the channels were originally created for older 11Mbits/sec.) You are showing at least six other Routers in range. Using the 2.4GHz range means you are trying to shout over those other Networks. This will work for a while, but when they are active, you will slow way down.
Slowing down appears to be what is showing with your updated Option-WiFi screenshot. Your signal strength is still very good, but your Transmit rate had to be reduced from 130 to 104 due to inability to send without errors at highest speeds. You may still be getting errors, which would cause re-transmissions and slow your speed test results, but may not be so slow that the Transmit speed would downshift again (yet).
As an experiment, try running your Router at 5GHz n-mode only. Use it for a while that way. Maybe even run the speed Test. Then capture that Option-Wi-Fi data again and post it here. I would expect it to show a consistent higher speed (like 300), and not give you other problems. But if it cannot attain a higher speed, that may indicate a Hardware Problem.
I do not understand why your Mac does not move to the 5GHz band automatically.