That happens to the Twin Commanders with the flap and aileron gap seals, too. Somehow the air leaking around the hinge and up over the flaps added lift, making them land a little gentler and use a little less runway. With the gap seals, it was faster, but less gentle on landing and not as short takeoff.
Many/most flaps are designed to have air flow from beneath the wing over their top surfaces. There must be a reason for that.
Airliners have the best of both worlds with a Fowler flap, which fully retracts.
Am not sure that you ASI is sensitive enough to measure the increase in airspeed
I’d say 3kt is worth having but there is no practical way to do it on a metal plane – because you do need most of those items. On a plastic plane one could integrate the antennae into the vertical stabiliser and AFAIK that is what airliners have (those with plastic tails e.g. Airbus).
There was also this thread although not conclusive since, presumably, nobody tried changing rivets.
Sorry, but I’m sceptical about speed mods. The reason I say this?
Our club has two P28As, one which was bought in the US, has a glass cockpit and aileron / flap gaps / wing root fairings whereas the other one has steam gauges and no speed mods. I fly both regularly but I’ve not noticed any benefit in terms of speed when flying one against the other – indeed, when checking the data from Garmin, it shows both fly an average of 108 knots over an hour’s flight (including climbing / circuit etc).
So in my mind, it’s the aviation equivalent of sticking go faster stripes on your Ford Capri…..
The fitting of a ’ Black Magic Waste Gate’ to my Piper Arrow IV Turbo by http://www.merlynproducts.com/magicprofiles.html has increased my speed by 4 knots and reduced fuel consumption by 5%.
By far, the Best Speed Mod I ever made was trading up to a Lancair Columbia 300 – gained over 40 Knots TAS over-night ! 
I have read reports of soft flap gap seals freezing to the flap in question. I’d probably choose a type that comes very close but is not in contact.
