http://www.quanthockey.com/nhl/seasons/last-3-nhl-seasons-goalies-stats.html
Happy to rehash the search parameters if that pleases you. Of goalies who have started at least 100 games in the past 3 seasons, Mason is:
13/17 in GAA
T5th/17 in SV% - quite clearly above avg. and very impressive relative to your d-corps. Tied with Luongo for this spot, .001 ahead of 4 other big names. Not trying to downplay the impressiveness of the figure, just pointing out that there is quite marginal separation between the 3rd and 10th spots here.
13th/17 in SO's
17th/17 in W - pretty irrelevant in this discussion, imo
I am unconvinced by anyone who says Mason is top 10 in the league with the above in mind. His SV% is unquestionably impressive, but it is also marginally (.001) higher than 3 (4?) guys I think everyone takes ahead of Mason: Fleury, Lundqvist, Rask, and Bishop.
Your second point might be true, I have no idea as to the internal dialogues that led to Neuvirth getting traded for Johnson. But it seems a bit far fetched that a rebuilding team would willingly rid themselves of (what your fan base seems to claim is) one of the single best backups in the league (by extension a reasonably competent stating goalie). Why would they not just...start him less often and keep him for the future?
Anyway, my argument is not that Mason is garbage, as hyperbole isn't my style. I simply don't believe that Mason is top 10 - he is average, and it doesn't seem unreasonable to suggest with the above in mind.