I would agree with you, no player can be called a playoff performer or anything like that because of 17 games... Even at huge sample sizes it is not reliable to say one player can/cannot play perform because of past performaces.
However, look at it this way: we know playoffs are a much more physical and demanding game where space is scarce. Isnt that the type of hockey Eller plays? Would that not be where Eller usually shines? I would think so. He seems to be indeed a player who's playstyle fits playoff play. From there, the past performances matter, because they seem to confirm this theory.
While it is true that we do not know for sure, it seems to me like we do not have the assets in our lineup currently to replace the type of two-way, physical possession play that he brings as a third line center. That is the kind of play that has been proven successful come spring.
None of Plekanec, DD and Galchenyuk can bring the mix of size and two-way play he does, and while De La Rose looks like that, I dont think it is a good idea to go into the playoff with a 3rd line center who played 30 games at best.
There is nothing to suggest that size correleates to being good in the playoffs either (at least on a player by player basis). Guys like Daniel Briere, Mike Cammalleri, Martin St. Louis, and Brian Gionta have all had strong playoff runs in recent years.
Just as Eller's past results do not guarantee his future success, neither does DD's.
And when we talk about mix of size and two way play... I look at Tomas Plekanec who brings the two way play (And has absolutely shut down David Kreijci in 2011 and 2014 playoff series, despite Kreijci having a line with two huge wingers Lucic and Horton/Iginla) and ask the question, does size matter or does talent when it comes to two-way play?.
The reality is this... Does the game get tighter checking in the playoffs, yes, a little. But really, five vs five scoring only changes a small amount. What really changes is that
1) PPs go down, as refs seem to swallow whistles.
2) PP% goes down, as teams are better able to game plan for their opponent's PP than they can in the regular season.
So you want to evaluate if a player can perform in the playoffs.... forget about size and ask yourself this. Is he able to produce offence five on five over large sample sizes in the regular season.
The reality remains that DD produces more offence than Eller even when we get rid of PP points.