The recipe you have selected will work I'm sure (I assume you mean 500g Dark DME not 50g), although perhaps you could improve on it. :thumb:
There are plenty of recipes for boosting Coopers stouts, look at the reviews here
http://www.thehomebrewforum.co.uk/showthread.php?t=17817
and here
http://www.thehomebrewforum.co.uk/showthread.php?t=17818
My recipe is kit can plus 500g golden syrup, 500g Dark DME, 100g dark muscovado sugar plus three espresso shots of strong Italian Roast coffee, and brewed to 19 litres, to keep it all in the FV. As I said elsewhere I prefer the Irish to the Original but its all down to personal choice. And there is no need imo to change the kit yeast.