For the type of texture that you would get at a sushi restaurant, try the Japanese way of tenderizing octopus. A good description of it is in ‘Japanese Cooking, a Simple Art’ by Shizuo Tsuji. It’s an old cookbook, but probably the most valuable one I have on my shelf. In my edition, the process is described on pp 248-251. Basic steps are:

1. Clean the octopus, inside and out, by kneading it with grated daikon radish (or salt).
2. Plunge the octopus in lightly salted water 2-3 times, mainly to get the tentacles to curl up nicely.
3. Boil uncovered (medium heat) in the same water for 5-6 mins.
4. Take out and hang it up somewhere to cool down by itself.

Size also matters. The book says 675 g is ideal for tenderness. The kneading and the slow cooling also help. You’ll need about 4-6 cups of grated daikon for a 675 g octopus.