Of course you get performance enhancement from frame plugs. By sealing off those cut tube ends you're eliminating those little vortices that are created when air travelling at speed passes over the open tube end. These vortices disrupt the air flow over bike and rider creating additional drag and thus reducing top speed. Furthermore at exceptionally high speeds these vortices can create lateral thrust which can induce gyroscopic eccentricity which to you and me means a 'wobble'. In extreme cases this can lead to the rider loosing control and falling off. I know it's happened to me before on track days but that was of course before I got frame plugs!
It's so obvious I'm surprised I needed to explain that to you!
So get out now and buy those plugs!