Well, it's not in the high BTUH 2nd stage much...The 24x30 return is already there and unless I rip it out and alter the wall and the ductwork it remains the bottleneck so to speak. (when in second stage)
However; how difficult would it be to make another 24X30 (or 24X24) filter rack somewhere in the conditioned area?
A lot depends on how much depth there is between the filter & the return area before entering a RA duct; we also need to know those factors.
Is it a rectangular RA duct within the wall?
The duct area could also be a problem...
The two (2) RA Filter-Grilles would improve the filtering process & improve airflow efficiency.
You wouldn't have to change filters as often, & it would really reduce the pressure-drop during the filter loading process...