I think his calmness would have disappeared if he had ever found out about the AR. Perhaps it is better that Angel doesn't know.
I think him finding out would be really interesting, or at least - has the potential to be if handled right ...
I think it would have been very hard to write him discovering this and have him getting angry without making him look like an insufferable chauvinistic ass - especially if Buffy was in the scene. After all, Spike didn't have his soul then and if anyone should understand the difference between the demon and the man - it's Angel. And if anyone has no leg to stand on when it comes to judging or forgiving the man for what the demon did ... it's Angel.
Xander's anger in the moment, finding her in the bathroom, is understandable. His telling Dawn about it days later is unforgivable. In one, he is reacting to the visible sign of his friend being hurt, in the other he is overriding his friend's wishes and privacy to spread around his anger - showing he actually cares more about his anger than he does Buffy's hurt.
So with Angel, so long after the fact and with Buffy clearly moving on and even choosing to keep Spike in her life, where does the anger come from? That a soulless monster hurt her? Well ... Angel's been there and done that. He's perfectly happy for Buffy to forgive
him. Expecting one rule for him and a different one for Spike would make him an ass.
Being angry whilst Buffy has moved on shows a lack of respect for her decision making and her agency. It rakes over something painful so he can have a catharsis without caring that the actual victim has had that catharsis already and the last thing they need is a rehash - like with Xander telling Dawn, it prioritises the man's anger over the woman's hurt and her right to deal with it how she chooses. And God forbid that he tell Buffy that she shouldn't forgive Spike - there would be no coming back from that jackassery, telling her how to feel, trying to make her decisions for her ... Angel does have a bit of history doing that. If he put any anger or judgement on Buffy for getting over it and moving on, telling her she was wrong to do it, that she needed to cut Spike loose, then that would be truly unforgivable.
Plus I think any kind of 'how dare you' anger aimed at Spike - beyond the rank hypocrisy coming from the man who is sometimes Angelus - would carry shades of 'how dare you touch
my Buffy'. There would be a sense of entitlement to it - an entitlement to Buffy, and outrage because it was something that belonged to him that got damaged. It's hard to explain what I mean ... I guess, Angel knows Spike has raped people in the past, Angel knows Angel has raped people in the past, Angel knows Spike and Angel have tag teamed on raping people in the past ... he doesn't walk around a seething mass of anger. So to get angry about Spike attempting to rape Buffy - when Buffy has moved on and openly admits to caring about Spike and wanting him around her - doesn't come from the same place as Xander's gut reaction to seeing her in the moment, but must come from a sense of ownership of Buffy and anger at someone else touching his things, IYSWIM?
Now, yes, all people get more angry about the mistreatment of people they love than they do of random strangers (not saying anyone doesn't care about the mistreatment of random strangers, but it is more visceral when it is a loved one) but I think that does come from the fact that people do feel a sense of ownership over the important people in their lives - after all, we use possessive pronouns to describe our relationships with people, it's only reasonable that that linguistic quirk would affect the way we feel.
But of course, just because we feel ownership doesn't mean we have ownership - and hard as it is, we have to step back and let the people we love make their own decisions and fight their own battles. It is recognised that anyone getting too involved, making judgements, putting on pressure and making decisions for the people in their life have crossed the line into unhealthy behaviour.
So it's really important that we get past this idea of defending someone's honour as being inherently romantic, or getting angry on behalf of someone we love for a past hurt is justified - especially if they are no longer hurt and have moved on. (Even if they're still hurt - getting angry doesn't actually help them, it isn't a proactive way to show you care, it's a way to make yourself feel better by getting all that negativity out. But that's not what they need - so it is selfish.)
It would have been a great BAngel moment though, to have Angel find out about it and instead of giving into his own feelings being immediately all about Buffy - how was she? what did she need? could he do anything? And have him ask why she keeps Spike around - to which of course she would show him some hard home truths about him and Spike being in the exact same boat. That would have been a huge moment for Angel's heroism and to prove how much he loved Buffy, though I think the writers would have probably gone for the drama of having him get angry.
It might have been interesting to have him find out in s5 of Angel, where he could go through his selfish rage without putting anything on Buffy and then come to realise the dark truth of the matter. it would have been a good learning curve for him and an interesting step in the ongoing love/hate eternal sibling rivalry that is the Spike/ Angel relationship. But there was already so much going on in s5 that I don't think he needed that lesson on top of everything else.
I'm currently writing a complete s6 fanfic (non comic compliant - have not and never will read) I might include him finding about Seeing Red later in the season and going from selfish righteous anger to a sort of grim understanding and examination of his own misdeeds, it would make an interesting challenge to write.