Just a little note that may help you with the performance drop when changing to borderless window depending on your gfx card certain settings may be different when running in both modes i.e. in full screen gfx settings are taken from game and when running in borderless window the settings are taken from your gfx software i.e. catalyst control center for ATI users.

What you want to check is the settings in your gfx card software and make sure that vertical sync is disabled and that you are not forcing AA on this may help with the performance issues.

Personally I would just play with the settings till it worked out right or set them all to be set by application.

Hope some of that helps.

Althire.