Why are you bothering to change the yeast? Coopers supply a specialist yeast for this kit.
And you don't want to add a 'tin of treacle' in my view unless a) you really like treacle and/or b) you have done this kit before and know what it tastes like unadultered and want to experiment. If you use that amount of treacle you will likely kill it, and overwhelm any of the coffee infusion whether it cold or hot steeped fresh ground or even, perish the thought, instant. Coopers Irish Stout is certainly one of the better kits out there in my view having done it a few times, and all you need to do is subtly enhance it. All I did to mine was brew to 19 litres, use 500g dark or extra dark DME, 500g golden syrup, 120g dark muscavado sugar and add four machine made measures of strong espresso, and found that hit the spot for me.
