It's hard to say how critial the space is above the filter. You may get a whistling sound as the air tries to rush through the constricted opening.

Have you considered using a 2 inch filter?

If whistling sound is the only downside (ie. not an extreme difference in strain on the blower) then I'll just order one of each size and see what works best. I assume the 2" will just need replacement more often.

Thanks

UPDATE: I found another site explaining in more detail why I should avoid the 4/5" filter if I don't have the required 7" total space above the grill (really = 2 5/8" minimum above the filter before the flex duct starts).
http://hvac-talk.com/vbb/showthread.php?t=964082

The OP there has *exactly* my scenario with same filter box measurements and space available.

I guess I'll go with the 2" for now: I should've had them enlarge that box when I had them change out the ducting.

I was originally recommended to use the Honeywell F40CR 4" filters by ZippyHvac in the thread here . There appears widespread acceptance that these 4/5" filters are excellent, better than the 1" Filtretes, at least for their primary purpose: protection of the unit's coils, as they can collect a lot more particles w/out requiring often change-outs and do so with less increase of static pressure.

The 3M filtrete filters do a good job of filtering but are very restrictive. You are better off with a cheaper filter.

The 2 inch filter is still am improvement over the 1 inch. Even a good 1 inch filter is OK if you check and replace it on a regular basis.