I said something similar a while ago. Although ST performances can be outliers that aren't good predictive meters for things to come, Kilian was down right dominant during ST last year. He was averaging 98 or 99, and would occasionally hit 101 now and again even when throwing 3-4 innings in multiple starts, and he was doing so with an easy and repeatable throwing motion that didn't come off max effort. Furthermore, he was locating his pitches well, especially that overpowering fastball. He was just blowing people away. He previously didn't really have an out pitch, but his fastball started to become one, and it played well with the change of velocity with his slider making that pitch also more effective. Unfortunately, then he got hurt in his last ST start.
https://x.com/WatchMarquee/status/1761142022943990036/video/1
According to his Triple A stats after he came back from the IL, it looks like he hit 99. 6 mph or so in one of his starts, but when he returned to MLB action and picked up a couple starts at the end of the year, he didn't seem to quite have the same velocity he had earlier in ST (he was only averaging 93.6 mph - the same velo he had in 2023), but it's also not as if it entirely disappeared, which isn't terrible after recovering from that injury. Even at 93.6 mph, Baseball Savant gave his FB a +1 in run value. If he shows up consistently throwing 4 or 5 mph harder, like he was in ST, you'd have to imagine he could be pretty effective.
For me, he seems like our team's dark horse candidate on the pitching staff. If he comes back looking like the pitcher he was during ST last year, I think he can force his way into the starting rotation.