Let's start small. Since we are talking about algorithms (better yet, about programs of a universal Turing machine), what about if two programs can match the same input on the same output?
Would that suffice as a definition of isomorphism, even if they have wildly different resource usages (including time)?
What if they don't output anything?
I previously posted this question in another discussion, but it didn't get any replies so, since I now have enough karma, I've decided to make it my first "article".
Any takers?