I'm also getting the Marvel cab at some point and may mod it (I don't want to touch AfM). ETAPrime shows lots of good small computers but never tests Visual Pinball from what I could find. What computer/specs do you use for your cab?
One suggestion for White Water. Try disabling some of the special effects, such as the ball trails. This may free up some much needed processing power and get rid of the lag.
As for updates, from what I've read they will be done via USB device.
As opposed to releasing the product after they fixed these issues that already existed? That isn't particularly cool. In general, anyone getting a table like this will be savvy enough to handle the USB updates.The main caveat: It could be several months before the firmware update is issued since Zen wants to fix all known issues with one update -- if possible.