I think you are right Sly it does depend on what car you get into. I think Lewis Hamilton was always going to be top of that list but being a Mclaren driver certainly helped - not that I'm doing down his acheievment.
It should be pointed out that Hamilton, Kovi, Kobi and Buemi are the only one's on that list that beat their team-mate in their debut year. Again though it has to be argued that beating Alonso and Fisichella in the case of Lewis and Kovi holds more weight than Pedro De La Rosa and Bourdais/Alguersauri in the case of Kobi and Buemi.
I wondered if the length of time in GP2 has any relation to the impact as was suggested earlier about Valsecchi so did a table for that too:
Lucas Di Grassi 3 and a half seasons
Pastor Maldonado 3 and a half seasons
Vitaly Petrov 3 and a half seasons
Karun Chandhock 3 seasons
Jerome D'ambrosio 3 seasons
Romain Grosjean 3 seasons
Charles Pic 2 seasons
Sergio Perez 2 seasons
Kamui Kobayashi 2 seasons
Nelson Piquet Jr 2 seasons
Bruno Senna 2 seasons
Timo Glock 2 seasons
Sebastian Buemi 1 and a half seasons
Nico Rosberg 1 season
Lewis Hamilton 1 season
Heiki Kovalainen 1 season
Nico Hulkenberg 1 season
It does appear that the drivers with less time in GP2 have made the biggest impact on F1. It should be noted that had Grosjean not taken the Renault drive in 2009 he probably would have been in the top 2 in the 2009 GP2 Season and may very well have landed a driver in F1 in 2010 so his 3 seasons (2 half seasons) was prolonged a little by circumstances. So if Valsecchi did get an F1 drive next year he would be the most expereicne GP2 driver to be promoted ever.
It does seem that the cream rises to the top but maybe the bottle of milk that is GP2 has been gulped at so much there is currently not much cream left.