One thing to remember... the team & crew chief make a huge difference.
There is no way an open-wheel driver with minimal Nascar experience could hope to be competitive in a team that is barely competitive with a Nascar veteran.
Examples:
Hornish has had some decent perfomances but sits in the bottom 3rd in points, while his 2 first-rate teammates sit mired midpack, despite a win each.
Montoya has actually out-performed his teammates, but usually runs mid-pack, just like Ganassi's Nascar team usually always has. His former teammate Franchitti wasn't so lucky, as Ganassi shut his team down due to lack of money... a pretty sure sign he never had a chance in the first place.
AJ Almendinger looked just as incompetant as the team he was driving for last year, but this year, they have hired experienced & skilled people and turned things around. Both AJ & Scott Speed have had good results this year, as they are being helped by people who know what they are doing.
Right now, Nascar is too competitive for ANYONE to shine much above the level of their team.