I skied a lot on XRC800 in both 170 and 163 lengths over past 3 years and the IC180 before that (basically same ski). I am both significantly taller and heavier than you.
The XRC800 is a bit soft for a more aggressive skier at the 200 lb mark especially in the 170 length. However when I ski the same ski in 163 the softness is not so apparent and it is a great all round ski. Also in 163cm it has a tighter turning radius at 12 meters and is a whole lot of fun.
That is my preference. However I will state that the softer Head skis all ski better in shorter lengths as the shorter the ski the less the stiffness is a factor.
If you really want a 170cm ski consider the XRC1200 as it is far more suitable to someone of your size.
Just for background my regular ski in 2007 was the XRC1400 (I also like the 163), in 2006 was iSL 160cm, 2005 XRC1400 (163), 2004 XRC 163cm. During that period I skied the IC180 in 163cm and XRC800 in 163cm regularly. I tried the XRC800 in 170 and did not like them at all (too soft). The XRC1400 and 1200 are nice in a 170cm but where I ski most of the time I like the turnier ski and hence the shorter length. Also skied a number of other skis (my 2008 skis are SS Magnum and boy are they nice!)