Just to add my 2ps worth vs the M135i too, list vs list, the standard spec is quite similar between the 2 cars - engine aside, the 135i you seem to get sat nav for nothing, but have to pay extra for keyless entry and vice versa for the GT86, a few other bits but nothing to shout about, of course you could argue about quality, perceived quality etc, again I think that will be *** for tat, better quality materials in the BMW certainly but I'd put a pack of haribo on the one made by Subaru being better in the overall build quality stakes.
List price vs list price there is £5.5k difference, if you start to try and do discounted price vs discounted price I can only go by my own experience which is that I bought my car for 22.5k, 2.5k/10% discount, unsure of what you could get a M135i for but I imagine the delta will still be a similar amount. Will that difference buy a faster car? 100%. Will it buy a better handling/more fun car? Much more subjective and difficult to say, depends on what you plan on doing with it I suppose.
I personally think that there will only be a small overlap in those who will look at both cars seriously, the price difference I believe is still enough of a gulf to see to that.