Here's a post/thread that covers why scorers in the East had a slight advantage over scorers in the West in the last decade-plus.
http://hfboards.mandatory.com/showthread.php?t=1494673
Based on the numbers presented there, if you are comparing St Louis to Iginla, you can give Iginla a 3-5 point bump per season right away based on Iginla having faced against better defensive teams on average.
There's also the more difficult to quantify factor of the role and ice time that St Louis received in Tampa. My sense is that star forwards in the Eastern conference and especially in the Southeast division have tended to play more shifts and longer shifts than their counterparts out west, which have resulted in additional opportunities for them to score points but haven't led to a lot of team success.
Take the ice time leaders for forwards in the 2007-08 season.
NHL Hockey Players
St Louis led all forwards in overall ice time. Subtract SH ice time and he's still second behind Ovechkin. 7 of the 8 forwards with the longest average shifts played in the Southeast division, and 18 of the 20 forwards with the longest average shifts played in the Eastern conference. Yet the Western conference was considerably stronger. According to h-r's SRS metric - which is basically scoring differential adjusted for strength of schedule - the 10th best team in the West was slightly better than the 3rd best team in the East that year.
So again, what were all those gaudy point totals being scored in the Eastern conference really buying their teams?
Look at the performance of St Louis when he finally went to a structured team that rolled four lines. He went from a point per game in Tampa to half that (23 points in 44 games) with the Rangers.
I think Steve Yzerman and the Team Canada staff agree with this viewpoint to some extent. They left St Louis off Team Canada in 2010 and 2014 and picked several lower scoring forwards from the Western conference. And when they ended up bringing him in 2014 he didn't really look that good.
Of course everyone probably has an opinion on this topic because we've all seen his career, but that's my take. I'm certain there's a small effect related to strength of schedule, and I think but am less certain that the team system/role aspect played a larger role. YMMV.
(I remember Sturminator comparing St Louis to Doug Bentley at some point during ATD discussion in terms of them sharing strengths and weaknesses - both were small, skilled playmaking wingers who were good backcheckers in open ice but didn't have the size to win the puck battles along the boards. You could also question whether Bentley was a guy who put up numbers on bad teams too.)