Crazy that you found an article where someone answered the exact question I asked haha. I was looking for more sustained success than 1 outlier season, but that list of names is informative. Here are the guys that appear on the list more than once (meaning sustained success with 2 pitches, * indicates more than twice):
Archer (we know he is a good comp)
Burnett
Kershaw
Jackson
Gio*
Kuroda*
Santana
Beckett
The question then becomes, are Newk's 2 pitches as good as those guys' 2 pitches? We can't compare all the pitches to each other easily, but we can compare FA velocity. Here are the average FA velocities of Archer, Gio and Kuroda over the 3 seasons they posted 3+ WAR as 2-pitch guys:
Archer: 95.0-95.9
Gio: 92.8-94.2
Kuroda: 92.1-93.1
Gio and Kuroda posted those average velocities back when the average MLB FA was more than 1 MPH slower than today, so we can mentally add 1 MPH to account for "velocity inflation".
Newcomb: 92.8
So Newk's FA is sitting a 1-2 ticks below the velocity those guys had on their FA to make being a 2-pitch guy work for an extended period of time. In other words, it doesn't appear his 2 pitches are good enough to sustain success by using only those 2 pitches.
It should also be noted those guys all used a 3rd pitch more than Newk currently does. They had a better FA than Newk AND they used a 3rd pitch more often to get these results.
I still think he needs to add a 3rd pitch, and I still think it needs to be a change. He probably only needs to get that change to about a 45 and use it ~10 times per game to be a truly effective SP over a decent stretch of time. It is certainly doable.
Oddly enough, I have seen Gio comps thrown around for Newk quite a bit. We should all be thrilled if that's what he ends up being.
I don't delve that far into stats but what exactly is FA? I'm assuming that's his average fastball? Cause I show 93.7 as his average fastball on fangraphs.