Would just query why you need similar gears to your road bike on a cross bike. You are likely to be travelling slower. Whether you are racing or not! Most of the single ring CX racers i know are using a wide ratio road cassette (11/28 ish) and something around the 40 tooth mark.
If you are just using it for gravel road hacking and the odd bit of tarmac, something like a 44/46 and 11/28 will give you a wide enough set of gears for 99% of what you are likely to do. And will use a standard road mech, with less big steps between ratios (my main bugbear with mtb cassettes in any application!)
You could even use an 11/29 campag cassette if you were that way inclined.
It was basically for the top speed. The bike will be used as a general mess around/commuter bike on road/gravel with a bit of recreational cyclocross mixed in during the off-season. The reason I was looking at existing ratios was to keep my high gear for descending, as I tend to spin out with my 53/12 as it is so didn't want to go any lower really. However there may eb some merit in what you say, off-road at least, regarding top speed.