The FDR model used on Q400 aircraft seems to be a Honeywell/Allied Signal 980-4700-027, also found on some jets, so at least that should not be a limiting factor. With all the Thales glass avionics and FADEC engine controls, it would seem digital data gathering would be easier than in older turboprop models?
Looking at this Japanese DHC-8-402 incident report and specifically the attached FDR data graphs (from Figure 4.1 onwards) there seem to be quite significant changes in parameter values inside a time frame of 1 second, which again indicates a high sampling rate:
http://www.mlit.go.jp/jtsb/eng-air_report/JA841A.pdf
so I would be rather surprised if the "sampling rate" would be only a "snapshot every 5 seconds".