I can understand MC wants to be stronger in preparation for his revenge.
But this kind of experiment in his skill so he can grasp the attributes and abilities of it, are a little bit showing off.
Some questions can be afloat base on the resulting battle against Noot Kelaine, which Sierra can't really kept a secret because its hard to believe in herself what she see.
If she keeps on being silence, then her loyalty and dutiful responsibilities will be questioned with. She needs a solid alibi in this case.
If that happen, Sierra will be forced to stick with MC because, she might consider to propose to investigate further the mystery of why a Noot Kelaine in that area to make shrouded the MC skill to anyone else under the guise of an "unknown" magic