If Sam had landed in a situation like Kurt did in 1999, there would be little if any debate about whether he as one of the best QB's in the league. With multiple HOF-worthy players around him, including at the skill positions and a brilliant offensive mind who specialized in coaching WR's and QB's and was creative at play design and exploiting mismatches, and propensity to take risks (and coach his players to do the same) it's hard to imagine someone with Sam's natural talent doing anything other than thriving.
Just he same, it's hard to imagine Kurt being anything more than incrementally better on the 2010 Rams, running Shurmur's Prevent Offense and having a diminutive slot receiver as your only reliable target.
It's all hypothetical, but Kurt landed in one of the best situations imaginable, while Sam landed in one of the worst imaginable.