Your ride will get extremely hard and also it will damage the suspension if you don’t drive on carpeted roads. Potholes will kill the suspension in this setup.
About stretched tyres you are risking your life because a stretched tyre isn’t sitting properly at the bead of rim and you are exposed to tyre failure. The lower diagram is of a stretched tyre to explain how it isn’t stable at the rim. The edge can come off easier from the rim
About overall diameter: 225/40-18 tyre is slightly smaller than stock size, the diameter of 235/40-18 is more matching to stock 215/55-16. Your fuel average will drop max 1 kmpl on 225 width and more than that on 235 though
My suggestion: go for 215/50-17 or 225/50-17 otherwise for 18s make sure you don’t drive on rough roads