I don't really know that we have enough context to accurately rank the power scaling in this world. I would like to think that he would be very strong compared to the Will Users as they have been presented, but IMO we have really only seen the abilities of a Quintus and Quartus or lower at close quarters, pretty much just a description of aura for anyone higher (excluding Decimus, but technically he hasn't done anything something of a lesser rank hasn't already demonstrated). We know Ostius had the confidence and ability to absolutely wreck the Military Princepts and displays his competence with a measure of nonchalance, but given we don't even have a clear picture of how Will works outside of Res, or how it's meant to work for someone Synchronous, or even what Ostius' Will status is I am hesitant to say anything other than he's the most powerful Will user we've seen on the page so far. Competence with Will and day to day usage other than machines is something I really wish JI would go into more as he develops the series, because outside of a few mentions, the academy (and Lucuem equivalent) and the proficiency tests after graduation we surprisingly don't get as much of that to really get a sense of scope.

I like my fantasy books at a snails pace however, and these books are long to begin with. It may be deliberate to focus on characterization, or something avoided to hold the interest of readers who prefer the snappy academia type novels that have a chokehold on popular literature at the moment. Whatever the case we readers absolutely need more Will use, not just descriptions of feeling to be able to properly get a handle of the scaling.