The F-gates are definitely not sufficient for the number of non-Schengen departures and arrivals. If we take tomorrow as an example (Tuesday is a pretty quiet day for non-Schengen flights), there are 27 flights to Turkey, England, Cyprus, Ireland, Qatar, USA, Lebanon, Thailand, UAE, Scotland, China, Serbia, and Ethiopia (plus ET to OSL, a non-Schengen flight). And remember that each widebody blocks two gates on F. With ADD, BKK, EWR, DOH, and ORD widebodies arriving between 06:40 and 07:25, you would basically block 10 gates on the F pier without non-Schengen gates 17-20.

Also remember that the F-gates (and 17-18) can be used for mixed origins, i.e. Schengen arrival and non-Schengen departure (or vice versa). Today, EI-SIB arrived from AGP at 15:22 as SK1804 at gate F30, and departed to LHR at 16:33 as SK1523 at gate F60. So, even if there's "only" 27 non-Schengen departures in one day, the total number of turnarounds that need a non-Schengen gate can be as high as 40.