I use Splashtop on my iPad 4 routinely to manage my windows 8 iTunes server PC. I have on a few occasions accidentally started playing an HD video via VLC when I meant to drag it into iTunes... and they usually play smoothly including audio through to the iPad.
Lag seems to be very minimal, in other words it never calls to my attention. My server PC is an i7 2600k with an SSD boot drive, but the system just uses the built in graphics.
Also, I've never tinkered with the settings to run Splashtop at a retina resolution. By default it appears to run at 1024x768 despite being used from an iPad 4. For my purposes that resolution, although not crisp, it's more than adequate for remote management.
If there's something specific you would like tested I'm willing to give it a try. Otherwise I recommend the app easily.