iMac WiFi speeds doesn't compare to other computers at home
I stumbled on this question because I'm having problems with poor wifi speeds after installing Yosemite on a July 2014 rMBP.  I don't get drops, but I get 5 Mbps speeds literally 5 feet from the router when I used to get in the 70s.  My iphone 5S, sitting next to the rMBP, is able to get 70Mbps. Even more confounding is that if I change the router to support only 802.11g, the speeds are at 54Mbps.

What I learned is that "MCS Index" value is significant--it communicates from the Access Point to the clients what coding schemes are in use.  There is a standard table defined by 802.11n, and you can view it here: http://en.wikipedia.org/wiki/IEEE_802.11n-2009

In your case--and in mine--an MCS Index value of zero shows up.  According to the table at the link above, that means the maximum rate available should be 6.5-7.5 Mbps. However, in your output, it claims you're getting 54Mbps, which seems wrong.

So, why am I answering?  Because in both cases, the evidence suggests that Apple's driver or the chipset is doing the wrong thing with the MCS Index value.  In my case, I sniffed wireless lan traffic and confirmed using wireshark that the AP is announcing itself with an MCS Index value of "7" (conforming to what I actually set in the configuration for the AP). Yet, my rMBP insists that it's a zero and sets the speed accordingly.  In your case, it appears to be recognizing the MCS Index value as zero but setting the speed incorrectly.

In short, I'm not sure that reinstalling your will actually change anything for you.