The problem with using the iPad over VNC is that there's bound to be some lag just by the nature of the problem; getting that much data over wireless isn't easy.
I personally use Air Display to use my iPad as a secondary display to my MacBook when I'm on the go, and it's nice to have the extra pixels, but I generally just use the iPad space for non-moving windows, such as reference materials and such.
To directly answer your question: no, you won't be able to get a smooth 60 Hz experience with an iPad hooked up to your Mini. However, if you're fine with a little delay then Air Display should work well for you - it allows you to use the iPad for mouse and keyboard input, which is pretty nice.
I should note that I haven't done this with the iPad as the sole display -- you will need another display at least for the initial setup.
Hope this helps!