New Lag Issue During Gameplay

It sounds like you have made a good effort in trying to sort out the problem, the steps you have taken are what is usually recommended t people who are having issues.
The fact that in your post, you mention that Guild Wars 2 is able to be played flawlessly without issue; means 2 important things.
1: you dont need to be worried about your system not being powerful enough
2: it is not a hardware or operating system problem.

Now, If you had just not done a fresh install of windows, it could be a possibility that a 3rd party program was using system resources while you were gaming, and thus reducing the performance of your rig. This is not necessarily a virus, it could be a trusted program including antivirus programs or even windows updating. Therefore, i would have advised you to open task manager during the game and see what other processes were using system resources and shutting them down. I do not think this is your problem however, but try opening task manager and see how hard your system is working in game and if there is something slowing you down, stop it.

Now, I believe it is most probably a driver issue. Are you using beta drivers or, more importantly, did these games work fine once before on your system before an update? Some drivers cause issues in certain games, or some games in themselves are badly optimized and don't work with certain drivers.
So my main suggestion, is change the drivers you are using. If you are on the catalyst 12.11 beta, go back to the last stable release, or if you are on the stable release go to the latest version of the beta drivers (i think it is at 12.11 beta 7 now).

Post back if trying the different drivers didnt help and we will continue trying to sort it out  
Hope i helped a bit