Try looking at the Greenheck or Ruskin web site. The submittals for their louvers all contain performance graphs of a standard 48" x 48" louver sample which show free area velocity vs. pressure drops.
The AMCA standard for louvers is to test a 48" x 48" louver (with the specific blade, head and jamb design) for pressure drop and water penetration.
As a general rule - most people consider this test to be representative of all louver sizes and it is but I always recommend that if the louver is short - less than 4 blades tall - use the pressure drop with a grain of salt.
Just a recommendation - drainable head and drainable blade with a jamb that channels the water away really gives you a lot of safety factor for water penetration.