I don't have an answer for the overhead question, sorry.
However, you mention having to turn on force logging - do you run a primary and
standby at the moment without force logging? If so, you probably don't have a
usable standby. See
for the gory details of what can happen.
In a past contract, I suffered from the problem documented due to a third party
running nologging in their batch code, and the system having been set up
without force logging (an oversight of a previous dba, I hasten to add!).
Turning logging on actually made little difference.
I would imagine that force logging would have less effect than the need to run
a backup after every nologging action to be honest. However that would depend
on what was actually done during the nologging phase.
Sent from my Android device with K-9 Mail. Please excuse my brevity.