Thanks to all; this is an excellent thread. I believe (speculation, but I hope intelligent supposition) the FBI's decision was principally based on some key factors:
a) The .45 ACP round is a proven stopper, whereas Bureau experience with the 9x19 (Miami) was less than ideal.
b) Springfield's HRT 1911A1 was certain to be extremely high quality, reliable, durable, and accurate (it's a "showcase" program for SA).
c) As a secondary weapon, the HRT's sidearm does not require the extra capacity offered by other semiautomatics -- plus reloading should be infrequent and instantaneous (given the capabilities of the team and their primary weapons).
It is interesting, however, that the Bureau's current standard issue firearm is a Glock in .40 S&W. This may suggest both that extra capacity is desirable for less-expert marksmen and that the 40 was believed t be the right compromise for penetration and expansion. Also remarkable is the Bureau's elimination of the 10mm, which certainly has "quantitative ballistics" superior to either the .45 ACP or the .40 S&W.
It is my understanding that Special Agents are permitted wide latitude in their choice of "daily carry" weapons; it would be very enlightening to know what is really carried by field agents day-to-day (for example, how many .357 magnums are still relied upon?).