Spam is indeed an ongoing battle, but that doesn't change the problem with the outdated rules or whatever it is that's scoring so much spam as ham. I'm having the same problem, with the amount of spam not getting caught by Spamassassin going up dramatically a few months ago. Like the OP, I ran a few through the spamcheck and found out that the scores should be higher than they are.
1) score in email 2.5, score from spamcheck 11.7
2) score in email -2.5, score from spamcheck 12.5
3) score in email 1.5, score from spamcheck 5.5
4) score in email 2.7, score from spamcheck 7.5
5) score in email 4.2, score from spamcheck 5.5
The Spamassassin people keep putting out new rules precisely because
it's an ongoing battle, but it looks like we're being hamstrung by not getting those rule updates. Expecting users to try to keep up in their own is ridiculous, and defeats the purpose of having a tool like SA in the first place.
I'm shopping for a new host because of the spam problem, but I'd really rather not go through the trouble of moving after so many years on LP. At the very least, let us enable full headers so we can see what tests SA is using to score emails. I have it set to "all" already, but I'm not seeing individual tests (for example, 1.2 MISSING_HEADERS).
The fact that they closed the other post without even addressing the issue doesn't give me much hope for a solution, though.