The short clip was just to show the FPS in the inspector without uploading a huge raw file. When I do more practical tests with fast, constant motion I can easily see frame skips, even with really low resolutions.
My disk is a SATA 3 SSD that I just got recently. I tried the test you mentioned: https://www.dropbox.com/s/ax01tz98mb2v7aa/Disk%20Speed.png?dl=0
If I capture with Quartz Composer instead of CamTwist I get the same result: smooth video in the QC preview window, and frame skips in Syphon Recorder.