RE: Flaps on a Flite Streak?
I've built and flown numerous Flite Streaks - including a Combat Streak w/flaps. What I've found on the Flite Streak platform is they don't really help. The FS was designed as a high performance model without flaps. Furthermore, a model with flaps is also more difficult to trim correctly.
You don't need flaps to make a model turn tighter. You use flaps to keep it from stalling when turning. Witness pure combat models. They all turn very tightly but none have flaps.
Additionally, I believe your .40 is way too much motor for the FS. My son and I are flying a FS ARF with no flaps and powered by an OS FP 20. We can fly it on 60' lines with no problem whatsoever. With 5 or 10% nitro fuel and a 9-4 prop, the FP 20 hauls our ARF-FS around at 5.1 second laps - plunty fast. Another disadvantage of your .40 it will make the model way nose heavy. Nose heavy planes don't turn well. To balance the heavier ending you'll have to compensate with tail weight. Heavier nose + heavier tail = poorly performing model.