Ive had quite a few hours in the new patch and noticed the optimum turning speed is pretty realistic in both of the Spits.
Its about 270Km/h for maximum corner angle, if you can get the 109 to drop his speed below this low enough and the advantage clearly goes to the spit IIA down to a point but if he maintains his speed, even though you might be pulling harder and slower, he will turn faster, quite a bit faster if there's a big differential in speed!
If the 109 pilot isnt just yanking on the stick and maintains his speed he will get you if you dont maintain 270km/h
If I remember correctly the corner speed of the early 109 E3 is slightly higher by 10Km/h.
Edit... yep just checked..
If your doing say 200Km/h and he maintains 235Km/h he has the turn advantage by 2 secs which is enough to get off a snap shot from the cannon!