BTree on-disk page ordering

The mention of the changes to the btree scan code in the latest weekly
news got me curious so I started looking at the 'executive summary'
(read as: README) of the patch changes for both the scan patch and the
btbulkdelete patch. If my understanding is correct, vacuum will only see
a speed improvement when an index's on-disk storage order has a low
correlation to index order. Is there any way to see what that
correlation is on a running system? I'm wondering if anyone has checked
to see what kind of performance impact a highly out-of-order index has
on index scans.
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461