here is the short-short explenation in terms of elementry physics:
yes, pulling the stick increases the lift (and also drag) produced by the plane. it does so by increasing the "angle of attack".
the lift (call it "F") is a force perpendicular to the area of the wing. by rolling the plane at an angle "a" you get a component of the force that acts against gravity - F*cos(a) = mg and a radial component that creates the centripetal force needed for the turn F*sin(a) and that is the component of the force you were talking about.
mind you that if you roll your plane 90 deg. like you said, your vertical component F*cos(90) = 0 ! therefor you'll be free falling while turning (in a spiral) but will get the fastest turning rate for a given speed (this will change as speed changes).
the G figure mentioned when refering to turnes is the ratio betwin that "F" force and the weight "mg". since in a level turn F*cos(a) = mg you get G = F/mg = 1/cos(a).
by pulling on the stick you increase "F" therefor also "G" therefor you can turn at a larger bank "a" without loosing alt. (not refering to drag and pull issues).
as for the turn radius, in a circular motion the centripetal component of the force must satisfy: F*sin(a) = m*V*V/r .
therefor if you increase "F" (or "a") and keep a constant speed ("V"), the turn radius "r" will be smaller.
note that pulling hard on the stick will also make your speed drop, and "r" must be even smaller to compensate. this is how a p-47 can "outturn" a spit in the short term.
if you are interested in turn-rate, substitue V=wr and you get: w^2 = Fsin(a)/(m*r) where "w" is [radian/sec] turnrate.
hope this answered your question. wasn't so short after all, sorry
Bozon