A stationary target isn’t going to be dodging anything, so its signature radius doesn’t matter for avoiding damage. A stationary (as opposed to merely webbed) frigate is easy prey for even a battleship, especially if said battleship is moving straight to it. Signature radius does matter when it comes to getting a lock.
A ship that will often end up stationary while firing in combat are Dreadnaught class ships. With the siege module active, this will enable them to do a huge amount of extra damage, immunity to ECM, near immunity to sensor damps, huge bonuses to repairs, and in exchange it can’t move and basically turns into a giant turret.
Given that a dreadnaught is going to be stationary when it is in combat and very few ships would have trouble locking onto a Dreadnaught, it seems signature radius isn’t relevant and a booster like X-Instinct (makes sig radius smaller) is useless. Since every subcapital ship pretty much has no reason to be stationary in combat, it seems the only ship that would want to be stationary in combat doesn’t care about its signature radius. Am I correct?