How ethical or useful is it to test the IQ of prospective employees in tech?

Between the non-creepy people who are kinda obsessed with IQ, the creepy people obsessed with IQ, and checkered history of IQ testing, I'd definitely avoid it.

Testing specific, useful, and uncontroversial mental abilities - such as working memory - would be reasonable.

Is there evidence that choosing employees based on IQ results in better outcomes?

Honestly, most places should just randomly hire people. They already don’t know what they’re doing, so it would save everyone involved a lot of time.

Being intelligent is good but it would be a mistake in my opinion to over emphasize this one attribute.

Lots of intelligent people are somewhat deficient in drive, motivation and ambition.

For example, one of the smartest people I know is a Mensa member. He works as a self employed home handyman. But you'd never guess it by looking at his house. He lives in a rundown fixer upper that he bought 20 years ago and it still looks pretty much the same.

Not useful. You usually hire someone to get something done, so you need to gauge their ability to do just that.

Intelligence is good for certain tasks. But the cleverest person doesn’t help if they don’t know how to make things happen, get along with the team, be on time and budget etc.

Like with "personality tests", if a company is using an IQ test as part of their screening then I question that company's processes and corporate judgement. It would be a red flag indicating to me that I would be a poor fit there.