I "solved" my problem with the Intel N-7260 on my new Dell. The story (jump to the third-to last paragraph if you want to see the solution immediately):
About three weeks ago I bought a Dell Inspiron 15r-5537 with an N-7260. My other pc's average about 55mbps speedtest.net download speed (Windows 7 and XP, TP-Link single band N router). But all the Dell could muster was about 17mbps -- although when I would insert a USB-stick wireless card it would immediately get 50mbps, and about 60 wired. Tried all kinds of driver and settings changes, and even some fiddling with router settings, and got it as high as about 23mbps download. I.e., maxing out at G-like speeds, even though the connection status to the router says N (usually indicating 130mbps like my other pc's). During all the experimenting some things I tried (that others recommended) actually made things a good deal *worse*, especially some router changes.
Anyway, Dell support at first tried to tell me that this is normal, and gave me some silly explanation about why I should *expect* a wireless USB stick to do better. When I persisted they had someone come out and replace the N-7260 with a new one. Result: No change.
Moved to the just released driver, which added maybe 1 or 2 mbps. Meanwhile Dell said the lack of improvement from the card replacement meant they needed to walk me through a system recovery to factory settings. (??!!!)
I was debating whether to return the pc (still about week left on the 30-day return policy), and decided that -- as a "last ditch" -- since the Intel Centrino N-6205 in my Lenovo Thinkpad W530 gets me the full speeds I should get, I would buy one of those and install it in the Dell myself. (Only took a couple minutes, but of course I followed the usual precautions such as removing power and battery. Just required carefully popping out the keyboard to expose the N-7260, and then carefully swapping in the N-6205 while ensuring the antenna connections and holding screw stayed as before.)
I wasn't real optimistic, but I powered up, got online, immediately got 46mbps download on speedtest.net. Then changed the card properties to match those I previously honed in on in my Lenovo, and the Dell immediately shot up to the (proper) 57mbps the next time!
So for me the full solution was replacing the N-7260 with a different Intel card. Why that should be necessary is perplexing. Maybe for some reason the N-7260 just doesn't play nice with TP-Link? I have my doubts that that has anything to do with it, but who knows? Anyway, I was happy with the outcome and wanted to share.