The ET value is measured from the centre of the wheel, so with a 17x8 ET40 it will not be different than for 15x7 ET40. But the line through the strut top bearing and bottom ball joint is not vertical. So if the tyre diameter is a bit bigger the intersection of the line with the ground also moves away a bit. For that reason ET35 for a 215/40 R17 will not be much different anyway.

What I fine strange though is the fact that the Escort Cosworth is running the same uprights/knuckles as the Sierra Cosworth 4x4, but has a ET of 25 instead of 40. So at least 1 of the 2 is not optimal, and I would expect that to be the Escort, as the knuckle was developed for the Sierra and copied to the Escort. And I believe the Escort is said to not have such great steering feel....

Anyway, back to your questions. 225/40/17 tyres don’t exist and 225/35/17 are rare as well. So the best option would be to have 215/40/17. The best wheel for that in my opinion is 17x7.5 ET35, as the tyre then sits straight with the rim edge. This will also clear the Koni strut. Brakes should fit in a 17” wheel, but it depends largely on the design of the wheel. Reyland should be able to tell you what combination will work.