Thats not a bad build at all you have their. only things you want to look at changing is the (1x8gb) or ram to (2x4gb) unless your upgrading in the near future and if you could id try put some more cash towards a gpu, maybe look at the gtx760 there great cards for their price.
Also the cpu you have their is great. i used to have one myself and they overclock so easily and are so stable too. i was running mine a 4ghz no probs.
Is the stock cooling on the case good as it is included with just two fans? The only reason for the 1 x 8GB kit it was cheaper, would i be better off with the 2 x 4GB?
Yes the case runs very cool. 2 x 4 GB allows you to use a dual-channel RAM configuration. Theoretically this is more efficient but the real world different is relatively small. Arguably not worth the extra ~£10 you have to spend at the moment.
But up to you. If you have the money then go for it.
Essentially it will be the same since a R9 270 is a slightly lower binned R9 270X. Once overclocked, it can perform the same as a 270X. The difference between the two is worth the cost of the cheapest 270 and the cheapest 270X.