I think Unterhausen is referring to the chain angle. As the freewheel gets wider, the chain leaves the front chainring at wider angle which may cause front derailment. Short chainstays make this worse. I don't know the width difference between a 6sp and 8sp set up but, it seems like it's a small change and you could probably get outer and inner chainring guides like the CX folks use to keep it in place. It might (same word Unterhausen used!) make grindy noises though.
Good luck!