I got the 4.3 update on my rogers s3 about a week ago and am having the same issue. My wifi signal on 4.1.2 was great. I have noticed my general reception is quite a bit better but I dont like the tradeoff overall.
Also, until I went in and turned off a bunch of crap (am not rooted atm) the battery wouldn't even last me half a day. I've fixed that but the wifi is hella annoying.