Spam from residential IP's should never reach SpamAssassin. The mail server should be rejecting the SMTP connection. SpamAssassin is something which deals with mail that has been accepted by a server: i.e. delivered. It shouldn't need to have any rules about residential IPs; what's the point.
Residential IPs are spammy, so if for some reason you've decided you're going to let SpamAssassin to handle them post-delivery, it would make sense to give them a high score.
That's an interesting heuristic. Given a host connecting from an apparent dynamic IP without matching forward and reverse DNS, we could take their purported e-mail domain (from where? SMTP hello? Or domain part of MAIL from?) and fetch the MX record to see if it points to that same IP address and use that as a whitelist criterion against being dropped as a residential IP. (On the hypothesis that they are trying to run an earnest self-hosted mail setup.)
However, if the host passes this check, and all other tests such that we decide to accept the mail for delivery (to be further processed by SpamAssassin), at that point why would we want to apply any score in SpamAssassin regarding the residential IP. We already decided to pass it.
