Huge framedrops with nvidia vrworks ue4 branch whit sli

I did want to try ue4 ShowdownVR, so i made a simple test with a single 1080 enabled, and raising r.screenpercentage to 400 give me a framerate of 35.

Switching to sli with two 1080 with same r.screenpercentage give same framerate but with lots more framedrops.

Since nVidia VRWorks branch of UE4 got the SLI VR in which the system use one graphic card per eye i’m expecting better performance, not worse; nontheless i’m not gaining any improvement in framerate, but also i’m dropping frames, almost ever from the same eye.

I’m using htc vive, and from steamVR i selected asynchronous reprojection over interleaved; in ue4 i checked all possibile vr-dedicated vrworks options (instanced stereo, multires rendering, lens matched rendering and single pass stereo).

Is there something i am missing?
Please help understand what’s going on here.