If you want to get a better Wifi connection, get a good router and plug it in to the one installed by Comcast.
If the grocery store makes a big deal out of having staff to carry groceries to your car, and how they sell 120 groceries, they had better be able to carry all 120 to cars that support it.
Wifi is extremely finicky, due to interference and the FCC's limits on legal transmit power levels. But you should be able to get same-room speeds of at least 180 megabits if you have a decent router and a client that supports it. The key things are you need to use a 5 GHz radio and (for distance) channel numbers >100. Many ISPs still give out 2.4 GHz-only radios, so performance will suck.
You could of course make the argument that you bought some combined cable modem/router/wifi AP box from them and it should "just work" - but you're just going to talk until you're blue in the face. Simply think of them as a pipe provider (not your IT support) and you'll be a lot happier.
What they provide is in general crap, and costing you money you can save off the monthly rental fee by providing your own equipment. This will also generally result in better performance.
I suggest buying a Surfboard 6183 (or lesser, depending on your area), and the wireless router of your choice. I recommend for simplicity sake the Apple Airport extreme or any quality 802.11n (or 802.11ac if your laptop supports it) router. Your network should look something like Cable from wall -> DOCSIS Modem -> Your wifi router. The only time you should be calling comcast is when the actual cable modem cannot obtain a lease and/or internet is in general failing.
Relying on Comcast to manage your CPE is just a plan for disaster. If your wired tests are getting 100Mbps, then the problem is on your end.
Either buy a Cat5e/Cat6 cable or change your wifi access point to use 802.11n. I am connected to my 2007 Airport Extreme at 300mbps right now using 802.11n.
This has more information on cat5 vs cat5e vs cat6 and their compatibility with different speeds / modes: http://serverfault.com/questions/107172/what-is-the-actual-d...
TL;DR almost any Ethernet cable you buy today will be cat5e or better, which will work just fine. Really old cat5 is technically compatible and should work too, but due to it having slacker tolerances for e.g. crosstalk, it might not work over e.g. longer runs or difficult situations (cable ran next to fluorescent lights, etc).
EDIT: If you want to know what speed your link-level has established, your OS should provide this info. netstat -e on windows or netstat -i on OSX / Linux should show the connection speed. In windows, you could also do WinKey+ r -> ncpa.cpl -> right-click active internet connection -> connection details IIRC.
Does anyone know if this OpenWRT works? I would love to push my speed to its limits. Comcast pretty much has a monopoly in my city and they are not very helpful.