This issue has been fixed with latest InstaVR.
Previously InstaVR had used "safe and fastest" anti-alias quality and texture filtering method by default, that's why the rendering result get relatively "fizzy" or "blurred" than other Gear VR apps.
InstaVR's default setting is updated to use "best quality" by on anti-alias quality and texture filtering, now.
Additionally, InstaVR supports more higher resolution such as 24576 x 2048, 49152 x 4096, etc. than 18432×1536 that is maximum resolution of other Gear VR app.
So user get much more crisp / sharp rendering result now.