I built Firefox 30.0a1 PGO/non-PGO unofficial builds from a same source code, using VS2013 Update 1.
SunSpider 1.0.2 (repeatCount=30)
-O2: 167.4ms (+/- 1.1%)
-O2 PGO: 177.6ms (+/- 1.0%)
-O2: 1291.7ms (+/- 1.3%)
-O2 PGO: 1341.8ms (+/- 1.7%)
I had built unofficial Firefox PGO builds using VC2005 for years. In many cases, PGO builds made with VC2005 had shown better benchmark results than non-PGO ones.
Messages at when PGO of VC2013 was applied to mozjs.dll:
548 of 43005 ( 1.27%) profiled functions will be compiled for speed, and the rest of the functions will be compiled for size
56726 of 352725 inline instances were from dead/cold paths
41822 of 43005 functions (97.2%) were optimized using profile data, and the rest of the functions were optimized without using profile data
11168873681 of 11168873681 instructions (100.0%) were optimized using profile data, and the rest of the instructions were optimized without using profile data