Fred looked into bootchart and discovered the Cooker package is just broken, and suggested I use the 2009.0 one for now, while he fixes Cooker. So I did, and here's the results. What Fred calls 'perceived boot time' - time to GDM up and running - is 0:17: bootchart. What Fred calls 'real boot time' - time to console login being available, i.e. all startup processes have finished - is 0:21: bootchart. Interestingly, I had to disable ntpd to get that reading. With ntpd enabled, real boot time is 0:27. I think it's because this system uses wireless, so ntpd doesn't successfully come up until the network connection is done (takes five seconds or so).
The installer guys claim to have fixed the ext4 issue, so I should be able to do an install to ext4 now and test with that - should be interesting. Obviously, I'll be doing a parallel install this time!