...And the results are...

...Interesting.

Here's what I did.  I decided to do a quick background comparison test using a piece of watercolor paper (with a gesso coating) versus Bristol board.  At the same time, I decided to test my new purchase - a Winsor Newton Series 7 size 2.  The only waterproof ink I had to test it with was my little bottle of Winsor Newton Indian Ink, so that's what I did....

The image on the left is the watercolor paper with gesso, the left is untreated Bristol board.
First thoughts?  I HATE inking on gesso - it's not just the feel of the brush on the surface, but it catches my fingers making the inking more "wobbly" as a result.  I used my Rosemary brush on the gesso, as I didn't want to ruin the W&N straight away.  I did use it on the bristol, and liked it a lot - but the ink!  The W&N Indian ink just didn't want to flow!  I normally use Sumi ink, which is gorgeous with a brush, but compared to that, the W&N ink felt like tar - it just didn't flow.  The upshot is I'm going to try to get some Pelikan ink (supposed to be really good with brushes), but I digress...

I did the background using the same basic method - that is, I got the page wet, and applied some diluted Golden Fluid Acrylic (Pthalo Blue), trying to get a gradual fade.  I obviously didn't succeed, but that was becausse I was short of time.  Both surfaces were much less absorbant than the basic watercolor paper (which I expected), but both behaved in different ways.  The gessoed watercolor page did have some texture, which stopped the paint from moving very freely.  The bristol board is much smoother, which lets the paint flow better.  You'll notice that the bristol has a more "watercolor" feel because of that - the quite stark color gradient is because I had to mop up the remaining blue paint on the surface as I ran out of testing time.  I think that given more time, the result would have been more pleasing.  It also leads to another point:  with the bristol board, I could lift the paint off more like I would with watercolor, so I could fix mistakes.

So what's the result?  Well, I prefer both surfaces to untreated watercolor paper without a doubt.   The thing is, I'm not sure if I'd prefer gessoed illustration board to Bristol Board.  Bristol is cheaper, more easily available, and takes ink better - I may well stay with that.What I might do is sand the gessoed illustration board a bit, and see if that helps...