I had my office switch to MERV 11 filters before the return to work. Here's what I can tell you.
1) Your filter box should be sized so that the airflow is at the very most 2.5CFM per square inch of filter. 2.0 is a more reasonable residential target, but my building's filter box is actually 1.0CFM per square inch of filter, which is perfectly ideal for higher efficiency filtration (it was engineered to LEED Gold standards, so that kind of makes sense). Find out what the airflow of your unit is, and if your airflow to filter area ratio is getting up there, expect negative consequences. tl;dr: many older systems just don't have enough filtration area to step up to a finer filter.
1a) Ok, so maybe you don't know the CFM. At my office, the filter box area is just under 3.4x the internal area of the main duct. If yours is at least 2x, you'll probably be good, so what's your duct size?
2) When it comes to 4" pleated filters, ALWAYS spend for the high capacity filters. Look at the pictures online and count the pleats. HC filters will have close to double the pleats, which all other things being equal means double the lifetime, yet they're never fully double the cost (don't forget to account for the time it takes you to swap filters).
