Just to jump in on this, i recently did the same move, went from diesel to turbo petrol, albeit a mapped FR not a Cupra R. I changed jobs and stopped doing 30k miles per year, so no longer neaded 50mpg+
Just by way of context, I'm 26 with 8 years NCB living in a low risk area and I pay about £500 per year (includes the map declared and 20k miles per annum and business). This was an increase of about £100 over my last car, really quite reasonable. I would guess that your FR is quite high anyway so the difference might be small.
Road tax is not that great an expense if I'm honest, yes it doubled for me, but in comparison to other costs, I cant get that bothered about it.
Fuel.... whilst I don't have a Cupra R, I imagine it is broadly similar to mine on fuel (same engine). It can do 30mpg on a run, i got 38 recently, but I get 28 on the run into work. I drive sensibly (not hypermiling) and give it the beans every now and again. You will spend a lot more on fuel.
Tires will be expensive and you should never cheap out on them. Again though, your FR probably has decent rubber on 17" so the difference might not be so great.
I'm not sure if this helps, but the long and short of it is:
Is the cost of doubling you motoring costs outweighed by your desire to have a Cupra R?