1) Although I have never flown the 757, I have approx 7000 hrs on the 767 and I have never heard using the method you describe to "stop wander" - and quite frankly, can't see how or why it would. It may be valid, but stick to the SOPs - they are there for a reason and you won't get shot on a route check if you don't vary from SOPs! You are sticking your neck out if you go off piste!
2) I believe the speedbrake does increase stall speed by a few knots, but the protections are such that unless you have a heap of bank on and are back under your manouevre speed for the configuration, then you are wasting your time worrying about it.