elcol
I don't have a flight manual to hand, but IIRC this data is not published in it (at least for the -140, 151, 161 and 181) and you have to look at the flaps 40 data and make an educated guess.
Again, IIRC,the approach speed is about +5knots.
If you need to know serioulsy (i.e. you are field length limited), I'd book a couple of circuits with an instructor, get a briefing and then take a view on the difference.
Have done many over the years, I don't recall it being dramatic, but then again I was using runways with 700-1650m length, so not really length limited.