I just couldn't resist bringing this pen into use a little early so I could compare the two Brown Tortoises. I think this pen is darker over all in its tortoiseshell pattern when compared to my M400.
I had to drag out the M800 IB nib to match the M400 version I received yesterday - I think the M400 is a smoother nib.
Just to complete the matches I picked I a Montblanc ink as well.
The M800 is a very broad wet rounded stub not a sharp italic. It runs to over 1.5mm in line width and is not all that useful in everyday use. It does however allow this (IMHO) boring grey ink to show a bit shading.