The info from MiG-21 manual and aerodynamics textbooks...
-deltawing has a low but long polare, unlike "conventional" wings - aka rectangle/trapezoide, which means, after some AoA - and it is not much, the lift does not grow anymore, but stays more or less the same, only the drag increases.
-with AoA, the aerodynamic center shifts BACKWARDS for highly swept delta (MiG-21 case, manual confirms)
-by manual, the plane is rather unstable per AoA - to explain shortly - "Cobra, Su-27 style"
-by manual, from 21S on, emptying the nose (last in order) tank means drastical shifts in CoG, making the plane even more unstable, usually too dangerous for landing - so it is basically a "last-resort" tank and during normal flight, you should not touch the fuel in that one.
As my school year is getting to the end, I hope for return to modding, possibly having a further look on the 21's.
Personally I think Vega's FM's are great - no doubts about that. But that is only at low AoA, like up to 10 deg, above that - well - it needs a touch. A big one.
Oh, and for a touche at our modding gods...
Using Lagrange's polynome for creating a polynome describing a curve from a paper manual is... not the best idea due to inaccuracy in the numbers we read from the graph. I know a better method