I ride an 853 Inbred and when I built the bike up I fitted it with a 70mm stem and it just felt wrong (having been used to an 80mm stem on my DN6 Inbred). I suspect that it just felt 'different' especially with it being a new build and that I should have persevered. Anyway, I fitted an 80mm stem and have used that ever since and it's felt spot on with my low risers rolled back ever so slightly.
I've spotted a rather nice looking silver Thomson stem but it's 70mm, would I notice any difference fitting this and just rolling the bars back to vertical (I can always nudge the saddle back if the reach doesn't feel right)? The only other difference is that it's 0 degrees rise (as oppossed to about 5 or 6 with my current stem), but it's over such a short length that I can't see this making much difference either.
I guess it depends on how tall you are and you arms aswell but on my 853 inbred I built it with a 60, and it was great but maybe just a tad short so I tried a 75 and hated it, my downhill mojo went completely, then I got a 65 and its the best
I reckon 10mm is a lot of difference, if you find 80 spot on then stick with it rather than getting a nice shiney stem. If you start moving the saddle back you are moving your weight distribution back