There's a reason the Handbrake devs are switching the presets to use constant quality. There is simply no comparison as long as you know what you're doing when you change the RF value. The only advantage, IMO, ABR has is that you can predict the file size before encoding.
Which is primarily why I use Universal with some custom trimmings for my encodes...I can figure each movie will end up in the 2-3GB range and I'm fine with that.
That said, do you believe the CQ default setting and single-pass encoding produces comparable results to, say, 2-pass with a static (say 2500ish) bitrate?