Thanks AEO. That makes wheel purchase decision really easy. Or should I move on, and upgrade to 10-speed? Nay, too costly.
Thanks everybody for your replies.
7800 and 7801 have tall splines because shimano found out that aluminum freehub bodies get gouged by steel cassettes with the 1mm splines. Tall 2mm splines are a 10sp only feature since 7700 used a steel freehub body and only needed 1mm splines.
7850 and 7900 use titanium freehub bodies with the conventional 1mm splines.
8/9/10sp freehub bodies are the same width, and the only thing that makes 7800 freehub bodies 10sp specific are the 2mm tall splines.
and no, you cannot interchange DA freehub bodies between 7700, 7800, 7801, 7850 and 7900, they're all different. You can checkout the exploded view of DA hubs at shimano tech docs to see what I mean.
Thanks