Though I'd like to see him develop a two seamer, though down the road, it's a good complement to a rising four seamer b/c the velo is often the same, the arm motion is the same, but the movement is so different.
In the case of Painter, a 2 seamer would be a change of pace to move the batter's eyes, not a bread and butter pitch. Bringing a 4 seamer down in the zone negates its biggest strength. We see this with Nola, where the two seamer has been the bread and butter, but developing a four seamer allows him to attack the top of the zone and move the batter's eyes.
However, refining Painter's changeup would be a much higher priority right now, it's a must pitch to deal with ML LH hitters.
I do think the cutter makes sense down the road for this reason, you want to change speeds along a spectrum as well as eye level, Colon was the most infuriating pitcher to watch b/c he basically threw FBs but would vary the speed from 85 to 95 and drive hitters crazy - you'd watch those pitches and wonder why they couldn't square up on him, but he was so good at keeping hitters off balance just by changing speeds.
So if the FB is in the mid to high 90s, and his other pitches are in the mid-80s, something in-between would help to disrupt hitter's timing and keep them from sitting on off-speed stuff if he's struggling with FB command.