If you read Reyk Floeter's recent Undeadly story, you'll notice he as well mentions that GENERIC.MP enables ioapic(4) and another method of interrupt routing.
(I myself own a few systems that actually get a performance boost by using the MP kernel, even if their non-MP systems.)
http://undeadly.org/cgi?action=artic...20080808071238
If you can, obtain a bsd.mp off a mirror.. and try booting with that, be thorough... see if it works with apm, if not.. disable apm and use acpi.
Good luck... 4.4 is around the corner, with a new ACPI implementation, perhaps it might work.
