The Stormer HVM is equipped only with Starstreak missiles, which lack a proximity fuse and require direct hits to destroy air targets. This makes it significantly more difficult to engage fast-moving fixed-wing aircraft compared to other SPAA systems at its current BR.
At BR 10.3, it is matched with vehicles such as the Osa-AK and ASRAD-R, both of which are equipped with proximity-fused missiles that make it far easier to score hits on fixed-wing aircraft. In comparison, the Stormer HVM is clearly inferior in practical anti-air performance.
For these reasons, I believe that assigning the Stormer HVM a BR of 10.0 would be more appropriate.