Some tests have been done on bottom bracket deflection to measure stiffness. Typically (though there are lots of testing protocols) this is done by clamping the dropouts into a fixture, and applying a static load to one pedal (cranks at 6:00) and measuring the lateral deflection at the BB. Obviously, it’s not just the frame/fork, the cranks, pedal spindle, etc. also factor in.
I don’t know of a test that has been done recently on tri-bikes; but Cannondales and Kuotas are generally known to be designed with a premium on BB stiffness.
BB stiffness has almost no impact on performance (i.e. there is no loss of forward propulsion), except for the rare case where the frame flexes enough that it creates excessive chain rub or unintentional shifts during hard efforts…
A couple of links:
http://www.sheldonbrown.com/rinard_frametest.html
http://www.renovobikes.com/testing/2009/4/18/lateral-stiffness-evaluation-of-8-frames.html