I had a similar experience with an older router (Linksys WRT610N).
I had 25/5 before with Comcast, and my router gives me 22/5 over WiFi, which I thought was perfectly normal.
Then Comcast upgraded the speed for all plans, and I got switched to 50/10. I reset my cable modem, and ran the speed test again. Still shows 22/5 over WiFi. I called Comcast and they said my apartment building has old wiring, and probably that's why. I didn't push for a further explanation.
A couple months back there were deals on cable modems (I was (stupidly) renting one from Comcast). I bought my own, and went to set it up. During the setup I need to plug my laptop directly to the cable modem. After the modem is installed I ran a speed test.
I got 50/10.
I was like "?!". I immediately setup my WiFi router again, and connect my laptop to WiFi. Ran a speed test and it showed 25/5. I plug my laptop to the router directly and I got 50/10.
So I am pretty sure the WiFi is causing the speed drop. I was using custom firmware (DD-WRT) on the Linksys and I thought I need a newer firmware, but I decided to tweak the WiFI settings a bit first.
[Disclaimer: Tweaking WiFi setting on your router improperly can cause performance decrease and dropped connections. Loading a custom firmware on your modem can turn your modem into a brick if you are not careful. You have been warned.]
I tried turning up the TX power on the antenna, increase the channel width, manually set the transmit rate. None helped.
Then I noticed one setting in the help file:
Frame Burst
The default value is Disabled. Frame burst allows packet bursting which will increase overall network speed though this is only recommended for approx 1-3 wireless clients, Anymore clients and there can be a negative result and throughput will be affected.
I turned the setting on, and I got 50/10 over WiFi. I usually don't have a lot of WiFi clients on at the same time, so that worked out for me. My guess is your new Netgear R6300 has frame burst enabled, or they have come up with a better implementation of the TX/RX algorithms.