Just in case anyone on here is going mad trying to fix poor wifi on a Nexus7.
Mine has worked perfectly for 3 months and then recently the signal has been appalling unless it's right on top of the router.
Forums suggest operating system upgrade is the problem but nothing I tried worked. Then I discovered a post that said the problem may be caused by the back of the case being loose. Sure enough in the top left corner there was a gap. I snapped the case back in place and instantly it's fixed. Seems that's where the antenna is.

Wish the fix for iPhone 5 wifi issues was that easy, had to drive and hour and a half each way to my "nearest" apple store to get mine swapped:-(