Below is a chart illustrating the change in completion percentage from college to the NFL for current NFL starting QBs (note that I included Alex Smith over Kaepernick because the change was so recent and used Vick over Foles so that Foles didn’t affect the numbers).
Admittedly it’s not the most elegant illustration, but we can quickly infer some useful information. (The left extreme is a drop from college to pro, while the right extreme is a gain from college to pro)
Note the x-axis values. Just one player (Drew Brees) managed to improve upon is college completion rate by more than 4%. Conversely, 7 current NFL QB’s have a pro completion rate more than 4% below their college marks.
The average change (not weighted for # of attempts) is -1.56%.
This year, the average completion percentage for the QBs included in the chart is 61.63%.
Upshot – With a college completion percentage of 66%, Nick Foles could suffer a decline of 5.28% and still be roughly equal to the average NFL starting measure. (Assumption – the included player sample is accurate for evaluating Nick Foles.)
Coincidentally, Nick Foles is currently completing 61.4% of his passes, or right at league average.
Caveat – The sample above is subject to several unavoidable biases, namely survivorship bias, as any QB whose completion percentage dropped by a very large amount would likely not be starting and therefore would not be included in the sample above (i.e. Jamarcus Russell, whose rate differential was -9.7%). However, huge declines appear to be relatively rare, and within the sample several of those suffering the largest declines are in no immediate danger of losing their jobs (Andrew Luck, Sam Bradford, even Mark Sanchez is proving incredibly resilient when it comes to keeping his job), suggesting that organizational inertia somewhat counteracts the survivorship issue.
There’s definitely the survivor bias you mention. I wonder what happens when you compare all those current starting QBs performance to the league average in their rookie years or first year starting. That would eliminate survivorship, but not necessarily for the fact that the pro offense has changed dramatically (college has too so it might be moot).