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In the US, defamation requires proof Perens intended to harm GRSecurity.

Well let's see. His blog is titled,

Warning: Grsecurity: Potential contributory infringement and breach of contract risk for customers

He's clearly issuing a warning (which may be false) to GRSecurity's customer base. That's getting right to the very core of GR's business.

Given his role as expert witness in other cases he can be expected to know what he is talking about. That means if anything he publicly states in such directed terms is found to be incorrect, it can't be anything other than deliberate.

He cannot assert that his opinion has zero weight in the murky world of How Licenses Are Interpreted By Courts, or claim to not realise what impact this blog would have. He is a businessman himself, and indeed runs a consultancy specialising in these matters.

So pleading no intent to defame (if his opinion is found to be incorrect) would be a difficult sell. He is not an untutored commentard like you and me. He would be trying to persuade a judge that all his prior involvement in FOSS court cases and indeed his own business are of no consequence when his words in this blog are interpreted.

In fact if he loses this one then there might be a queue of people looking to reopen previous court cases in which he gave expert testimony against them. And a lot of his own clients might start wondering why they bothered seeking his services...

That's a high risk strategy for Perens.

As it happens I think the kind of people who are likely to be GR's customers are going to be very wary and will have previously engaged lawyers to advise them as to the "safety" of using GR's product. That is not at all unusual when a business is seeking to incorporate GPL code into their project. The fact that GR has customers at all suggests to me that an awful lot of lawyer time has already gone into this, and that they're pretty confident about it.

Don't bet on Perens winning this one.