Have you put your entire spool filesystem on solid-state disk, or at least very high capacity multi-spindle RAID 1+0 arrays with high speed battery-backed controllers with large quantities of write-back cache?
I don't understand this language, sorry. We have just upgraded the I/O capacity of the server, and it's working fine with every other piece of software, no problem with postfix handling tons of emails for example.
But still when we're talking about a 15-60 seconds lock for each subscriber, even if we could have the hardware go twice faster, it's way too slow. That's why I believe only the database backend can help (or any other software modification). What's especially wierd is that probe sending is very costly, but personalization or VERP-delivery is not that costly.
-- Fil