I've made this a few times and this most recent time it did come out a little astringent, no obvious change in process although I did sub a different coffee malt for the Franco-Belges. I'm sure you could go down on the black, I know many folks find it harsh although it actually seems to come out smoother to me than equivalent amounts of roasted barley. I was thinking of next time trying a cold steep or adding the roasted malts just at sparge, ala Gordon Strong I guess. I've not tried it before but interested to see the difference. This one does benefit from a little aging, seems to bring out the chocolate as it mellows.
Please report back if you brew a version of it!