Britton's success hinges more on control (2.0 BB/9) of a primary pitch that moves like crazy. Higher velocity likely decreases the movement, if anything, and lower velocity certainly wouldn't impact the movement.
If this is correct then this would strengthen the case for making Britton a starter because while he'd need a third pitch he would already have two plus pitches. The way I see it, his slider already serves as a pitch that can be used to strike batters out so he'd need to resurrect his four-seam fastball to become a starter. However, this all depends on whether the success of Britton's sinker does depend on its velocity.Using data from ESPN's Stats and Information Portal, it is possible to view the results of Britton's sinker as a reliever when its velocity was below 94 mph, between 94.01 to 95 mph, 95.01 to 96 mph and above 96.01.
When Britton throws a sinker less than 95 mph as the last pitch of an at-bat then the result is a strikeout roughly 12.5% of the time. When he throws the sinker between 95 and 96 mph as the last pitch of an at-bat then the result is a strikeout 22.6% of the time. When he throws it faster than 96 mph as the last pitch of an at-bat, then the result is a strikeout 30.3% of the time. The faster he throws his sinker, the more likely it results in strike #3. This partially explains why batters’ wOBA is lower against his sinker when he throws it faster than 96 mph than when he throws it lower than that. The differences appear to be somewhat drastic as shown by this chart.
However, the main difference becomes clear when looking at the results of balls put into play. Britton has thrown 85 sinkers faster than 96 mph that were put into play and has allowed only 2 extra base hits (2 doubles). This is primarily because 71 of the 85 (83.5%) balls put into play have been either ground balls or bunts. He’s allowed 5 flyballs and only one of those resulted in a hit (a double). In contrast, when Britton throws his sinker at speeds lower than 96, he’s allowed 13 total fly balls in 210 balls put into play in which batters have hit 5 home runs and 2 doubles.
As a result, batters are averaging a .264 wOBA and .024 ISO when putting sinkers faster than 96 mph into play and averaging a .291 wOBA and a .093 ISO when putting sinkers slower than 96 mph into play. Simply put, batters are unable to do much damage even against mistakes faster than 96 mph while they simply punish mistakes slower than 96 mph. This chart shows his results at different speeds on balls put into play.
Britton’s sinker also becomes harder to make contact with when he throws it faster. When he throws it faster than 96 mph, batters swing and miss 18% of the time. When he throws it slower than 96 mph, batters swing and miss 12% of the time. Britton has a 23% called strike rate when he throws his sinker slower than 94 mph compared to a roughly 16% rate when he throws it faster than 94 mph. As the chart below shows, the data suggests that the benefit Britton gains from increased control and being able to throw more strikes by throwing slower is neutralized by the benefits Britton gains by throwing faster and making it harder to make contact with this pitch. Once one takes into account the fact that Britton gives up weaker contact when throwing the ball faster, it seems clear that Britton does benefit from his high velocity and that using him as a starter would weaken this advantage.
That stated, the results from this pitch thrown at even lower speeds are definitely acceptable. As a starter, opposing batters had a wOBA of .321 against his sinker while they only have a wOBA of .288 against it as a reliever. If his sinker can remain as effective as a starter as it as a reliever then it seems reasonable to presume that he would have two good pitches.
The real question is whether he could throw a third pitch and what impact that would have on his current arsenal. When he was used as a starter, he used four pitches: a four-seam fastball, a change up, a sinker and a slider. Since he’s currently throwing the sinker and slider, it would make sense to presume that he would try to add a four-seam fastball or a changeup. The problem is that there are a few relievers that rely primarily on a sinker, but most starters throw a four-seam fastball relatively often. Not only would he have to reintroduce that fastball to his arsenal but he’d have to throw it often. Presuming he’d be able to do that is awfully risky.