Does awaiting a non-Promise have any detectable effect?

await is not a no-op. If the awaited thing is not a promise, it is wrapped in a promise, that promise is awaited. Therefore await changes the execution order (but you should not rely on it nevertheless):

console.log(1);
(async function() {
  var x = await 5; // remove await to see 1,3,2
  console.log(3);
})();
console.log(2);

Additionally await does not only work on instanceof Promises but on every object with a .then method:

await { then(cb) { /* nowhere */ } };
console.log("will never happen");

Is there any detectable effect of awaiting a non-Promise?

Sure, .then gets called if it exists on the awaited thing.

Is there any difference in behavior one should be aware of to avoid a potential error?

Don’t name a method “then” if you don’t want it to be a Promise.

Any performance differences?

Sure, if you await things you will always defer the continuation to a microtask. But as always: You won’t probably notice it (as a human observing the outcome).