There is a problem with these so called maximum theoretical speeds of wireless routers. They are signaling speeds and they do not represent the speed of sustained data transfer through router, so even if those 54 Mb/s look higher than 15 Mb/s, that may not be so in reality.
Furthermore WiFi speeds are highly dependent on correct settings for both router and network card and sometimes even if computer is wirelessly connected at high speed to access point, some device in the area could be slowing it down.
Another thing to consider is signal strength and how it will affect computers. Sometimes computers will decrease speed of wireless connection if signal is too weak.
So to cut the long story short:
Just by analyzing, you probably won't be able to determine if the router is bottleneck on your network. My advice is to first connect a computer using wired connection to the router and do some Internet speed testing, in order to see what your speed really is.
After that, you should do tests on your internal WLAN to see if it can sustain such speeds.
Tool I like to use for speed testing is iperf (link to Window version). It is a bit hard to set up, but it will provide you with information on how fast your network is.
First step would be to test speed between one computer using wired connection and one computer using wireless connection. This should be able to simulate connection to Internet well enough and will provide you with information if the router is bottleneck. You could also do an Internet speed test on a wirelessly connected computer, but iperf will provide better results, unless modem-router connection is the bottleneck (and on such low speeds it usually isn't).
Next step would be to see what speed you can get between two wirelessly connected computers. It will probably be lower than the wired to wireless speed.
To run iperf, extract it to a directory on one of your drives and open command prompt. Navigate to directly where it is stored and run it. To go one directory up, use
cd .. and use
cd dir-name do enter a directory. You can also use tab to automatically complete names of long directories.
You'll need iperf on both computers for test, and one will ahve to be a server and one a client. On server run
iperf -s and on client run
iperf -c IPaddressOfServer
Do note that 802.11n access point will be affected by all problems affecting g access point, but will usually be affected to a lower degree.