How massive can a star be at birth?
I found a text that suggests a Population III star can have a mass between 200 and 10^5$M_{Sun}$. There you can read:


  "...This would apply for VMOs larger than $M_c$ $\approx$ 200 $M_{Sun}$. Stars larger than 150$M_{Sun}$ are termed supermassive objects (SMOs)...".


It seems supermassive stars like R136a1 (with masses in excess 150$M_{Sun}$, the accepted upper limit for a non-pop III star) can be formed by the collision of massive stars. 

So, It is not difficult to imagine, say, two 150$M_{Sun}$ monsters colliding and forming a 300$M_{Sun}$ object.