The GAMMAINV function from Excel 2003 had a propensity for generating NA error messages for high alpha values as the iterative process failed to reach convergence. It was therefore common to use a normal approximation if alpha > 300 (the gamma distribution tends towards the normal distribution for high alpha).
In 2010 Microsoft refurbished many of the Excel statistical functions, including the gamma distribution, now called GAMMA.INV. The iterative search algorithm have supposedly been improved.
Q1: Should the NORM.INV still be called in situations where alpha > 300?
I did some analyses trying to input extreme parameters that should/could lead to failed convergence. I found that GAMMAINV and GAMMA.INV returned exactly the same results (also with respect to errors). I expected that there should at least some minor numeric differences between them...
Q2: Does this mean that the GAMMAINV calls on the same algorithm as GAMMA.INV in Excel 2010?