Ok... Here goes. I wanted to clear up this static regain once and for all for the Rheem/Ruud Air handlers.
Per all brands residential air handlers installtion instructions. You should install the plenum the size of the opening. Yes this is true and this is the correct way.
May people dont follow point one. However, when the air handler is installed on a proper duct system it is irrelevent. If indeed the duct system is marginal (expecting a .5" ESP or more), then yes you can pick up around 100cfm by adding the static regain. If the duct system is crummy (expecting a ESP of .7" or more) then fix the duct issue. I have personally spent the day testing an air handler on various duct systems and until you reach .5" total static, the regain does nothing for you in the form of CFM. This is with a flow hood attached. At high static, its a moot point since you are starving the system anyway. Duct systems should be desinged to operate between .2" and .5" wc ESP. Follow manual D and you will not have a problem, do it with rules of thumb, and you get what you get, simply put.
Any air handler with electric heat strips should have 3' of non-combustable duct with no take offs. This is a safety thing should a heater disinigrate which is indeed the failure mode for electric heat. Horizontal and Downflow are obviously crucial since gravity is no longer a factor. See National Fire Protection Association 90A and 90B requirements.
ECM systems were not designed to overcome bad duct systems. They were desinged to maintain proper airflow in changing conditions at a more effeicient cost. Yes, a marginal duct system can be dealt with easily, a poor duct system may be dealt with but noise, and control become an issue. Even ECm's have a limit to RPMs and neat 1" they too will have to follow a curve.
The BHA air handler was a weaker motor and unless all was perfect they often had reduced airflow. That unit is no longer in production and has been replaced by the BHC in 2001 with a stronger motor. (900 RPM versus 825). The cabinet didnt change so neither did the thought process from the rumormill.
The flanges. The air handler is shipped with the duct flanges folded down for shipping and should be bent upright, even if the duct is not connected to them. Many mfg's do not have this but these do. The big orang label on the top of the unit clearly points out that these should be flipped up. Personal testing has shown anywhere from 10-25% airflow reduction if they are left down. Simply put, fold them up no matter where you attach the plenum.