Report back with the results. This is the sort of thing that not many ever bother to play with.

It would be interesting if you were to tape on some 2 inch long length's of kitting wool on the flaps outside of the prop blast and have someone view them through binos while it's doing slow flybys. If the slot is helping you should see the upper surface tufts pointing down to the trailing edge to a marked degree. If the slot isn't able to fill in the stagnation bubble on the upper side of the flap then the tuft will be fluttering strongly in the vortex flow. Try different angles to see how well it works out. At some point it WILL stagnate and produce more drag but up to that time it should be a slower flying speed with little power. Once you drop the angle a bit more and it stalls the flap despite the slot you may find that it requires a nudge more power to maintain height and/or flying speed.