i bought the 940 out of curiousity (thanks to this thread!) and while it bears the same "type of sound" or sound signature, the HD800 outdoes it in every aspect as a top flight phone should. not to say that the 940 is bad - in fact it is very good value for what comes in the packagaging, as well as a very modern sound signature (very "hifi-ish - detailed and very clear across a very broad sound spectrum), the HD800 is just more airy in the treble, has more timbre in the mids and bass, and has more and faster punch in the lower bass, and a much much wider soundstage and more comfortable overall.
its correct to say they are similar - fast,detailed and to me it is metallic or has that glare across the upper range that adds to its clarity and sharpness - its clear that difference is between a mid-range and a high-end experience. so if you like this sound signature and looking for an upgrade to the 940, the HD800 is definitely in the right direction. whether its worth it or not, its up to whether you can or want to afford it, as the law of diminishing returns sets in quickly when you get to any high end purchase.
To me based on the cans I own, very roughly, I'd say that in terms of value and sound signature, the 940 is to the HD800 as what the HD650 is to the LCD2.