Walnut Blasting: is it worth it?

I have a 12 year old Skoda Octavia mk3 TSI 1.4 with 155000 miles on the clock. It has JUST scraped through its MOT after repeated emissions failures due to carbon buildup. The mechanic has told me he doesn't want to see the car back next year as it won't pass again and cleaning the carbon deposit off wouldn't be worth it given the age and value of the car.

I've read a lot about Walnut Blasting to remove carbon buildup. Would it be worth it in my case or is the mechanic right, and I should just look to run it into the ground and get another car for next year?