Twigs is not a former RAF pilot, Twigs is a current RAF aircraft engineer.

Wings sweep backwards in modern jets to increase speed but not how you think. It doesn't make it any more aerodynamically efficient than a conventional wing design. You will notice all current fighter jets have some kind of 'sweep' in the wings, no matter if they can move or not. This is so because as they approach the speed of sound, shockwaves begin to form on the wings, if you sweep the wings you delay the point at which these shock waves form on the wings.

Folding S-Foils in space would not give you any kind of a speed incrase as there is no air to create any drag and so nothing to prevent you hitting high speeds. In atmosphere though that may be slightly different, S-Foils out should increase the amount of lift generated (more surface area) and so be better for slow speeds. Locking S-foils would give a smaller lift area but decrease the amount of drag (since we now have to large surface areas removed), since drag increases at twice the rate of speed increase due to atmospheric pressure, it makes sense to reduce drag as much as possible to better increase your speed.

In other words, if you double your speed, drag will increase by four. So, if you have an object you want to fly rather than increase thrust to increase speed it makes more sense and is more efficient to decrease drag.

I could of course have the facts a little wrong there, this is basic aerodynamics stuff, I last sat through that lesson 12 years ago

Bite my shiny metal Tie Fighter!