I'd have to agree with you there. I've usually run a 24AC in my DuoDock as the only display, but I doubt it was any faster for gaming resolutions than the Dock's Video. The entire DuoDock is one big multifunction PDS Card running on the PBX bridged '030 bus at 20MHz, IIRC.
NuBus Video is for high resolutions at higher bit depths. What resolution, at what bit depth, is your target mode for Diablo?