Relationship between chain tension and chain wear
The chain should be as short as possible and as long as necessary. It will usually shift better with proper tension. The tension shouldn’t affect chain life though the shorter chain will. Make a 104 link chain shorter by 2 links and it has almost 2% less material and thus shorter life. But that’s really a small effect.

I can dimly remember an article which stated that higher tension (they tested big chainring vs. small chainring) is actually more efficient, probably because there are less vibrations.