SakashiroOK, so I hopped into my Courier and investigated, and here's what I found:
If you enter Sirius with something in your cargo hold, you may or may not receive a familiar message in local chat: "So the rumors were true, good I found you first" or "Big haul, surprised you made it this far" etc.
If you receive that sort of message and submit to the subsequent interdiction, you will be attacked immediately, even if you jettison your cargo.
But it gets even weirder: If your ship is fast and you manage to low-wake out, you will after a few seconds receive that sort of message in local chat again. The NPC(s) will not register that your cargo hold is now empty, and will chain-interdict you.
This behaviour resembles the conditions I talked about with the cargo transport missions in Ceos/Sothis for the Fed rank grind. But certainly not just there. If you have accepted a high level mission, even if you haven't picked up the cargo yet, the pirates are going to wait for you. They either spawn a few seconds after you jumped into the system and are right behind you and catch up during the station approach. Or they come out of nowhere also in destination vicinity. Only rarely one tried to interdict soon after entering the destination system. In any case they'll greet you with one of those messages.
I didn't try to jettison the freight due to being unique mission goods. But the Pirates opened fire immediately anyway.
And the pirate names are recognisable, as in attached to a specific mission. As long as this particular mission isn't completed they will be there again and again.
My conclusions ended in a minimal, but strong enough shield and chaff. Submitting immediately and, once in normal space, perma-boosting and low waking does work. Shieldless does not.
I also found one funny method to sometimes avoid the interdiction whatsoever. At station approach I waited deliberately until 5-4s "distance" before throttling to 75%, thus overshooting and started a loop of shame. The persecuting Conda turned even slower so I gained (sometimes) enough time to reach the station.