Here's the go, winglets are a drag reduction 'fix' and will show a slight improvement in fuel consumption over a long range, high altitude flight, say 4 hr at 35,000 ft. So for your purposes, prolly not what you expect. A tip 'fence' might help a model prone to tip stall, but you say yours does not, so, prolly not much to be gained there, either. You say your model is a bit over the advertised weight, this translates to a slightly higher airspeed at the point the angle of the wing to the airflow will start to break down. This angle cannot be changed, regardless of the weight the wing will always stall at this angle, but lighter means that it will do it at a slower speed, which is what you want. If you want to make the wings 'more effective' then you could make them bigger...or the model less heavier...maybe a bit of washout might help, but given your description of the stall, unlikely...besides, adding more bits to the model will increase the weight a bit, making the stall happen at a slightly higher speed, not what you are looking for...I am afraid that the only way to lower your stall speed is to make the model lighter, or rather, reduce the wing loading.
Evan, WB #12.