It makes more of a difference to performance if its a forced induction engine but im happy paying a little bit more for premium fuel knowing its keeping the engine cleaner.
There was a test carried out about 10 years ago by one of the big car magazines and they inspected the heads and valve assembly of 2 cars (1 was a Civic Type R) and after 1500 miles on the higher octane fuel they re-inspected to find the engine a lot cleaner and a lot of the carbon deposits were gone. They also tested the cars 0-62, 0-100mph and in gear times before using the fuel and after the 1500 miles on the premium to find that the cars peformed better on the higher octance fuel.
Headers usually get heat wraped when new before they go on the the car so if you were intending on wrapping the OEM header you would need to remove it first. Might beworth 2-3bhp on an unmapped car but more like 5-7bhp when remapped.
Just placed an order to carparts4less for the front brakes. I used their promo code and got a tenner off so the total for Pagid front discs, pads and wear sensors came to just 89.90, very happy with that!
The obc is usually a bit optomistic so with a full tank reset the trip computer and after lots of mixed driving take note of the milage and brim the tank and note how many litres it takes. Only way to know for sure.
The theory is sound, alot of Jap cars heat wrap the headers/mainifolds to keep engine bay temps down so the engine is kept at optimum tepm and produces as much power as it should.
To be honest you need to do more frequent oil changes than what the service shedule indicates. If it was my car I would be changing the oil and filter every 7-9000 miles.
A higher octane fuel won't do much to increase performance on a naturally aspirated car however it does make a small difference. Also im led to believe that the higher the octane rating the cleaner the petrol burns so it results in less deposits building up in the engine so your engine peforms as it should do.
Not sure about the dpf to be honest. I think some people get them removed by tuners just before a remap as a preventative measure because if they go wrong its an expensive part to replace.