I understand this, and as a young system, you would potentially have a lot more rocks affected by the proposed gas giant, but as you also point out, any un-bounded material should have already been ejected from the system. It is difficult, but obviously not impossible to change parabolas into hyper parabolas to enable these kind of speeds, but they obviously got close enough to hit the roche limit, or simply dissolved like the Christmas Comet of 2014.
Planet 9 is also theorized to be near 90 d(edit:30d) to orbital plane also, so tossing things out where we aren't looking for them is another hazard in itself. I think the orbital plane of galaxy is out where Pluto is now, (because of the diffuculty of finding secondary targets for New Horizons was made more difficult by background clutter from MW) and 9 is another 40d around the orbital plane, so with a (edit:15k) orbit, there is not a great chance it is going to be relevant in the double influence scenario.
I was tracking these runaway stars for a SF story i had in mind, but this is the closest one i have heard of yet, and the ArXiv paper describes one that also passed thru 2.5 mya.
Gliese 710 will pass the Sun even closer
Close approach parameters recalculated based on the first Gaia data release
http://www.aanda.org/articles/aa/abs/2016/11/aa29835-16/aa29835-16.html
Close encounters of the stellar kind
https://arxiv.org/abs/1412.3648
tl:dr article
http://www.businessinsider.com/star-hurting-towards-solar-system-2016-12\
"Gliese 710 is about half the size of our sun, and it is set to reach Earth in 1.35 million years, according to a paper published in the journal Astronomy & Astrophysics in November.
And when it arrives, the star could end up a mere 77 light-days away from Earth — one light-day being the equivalent of how far light travels in one day, which is about 26 billion kilometers, the researchers worked out.
As far as we know, Gliese 710 isn't set to collide directly with Earth, but it wil be passing through the Oort Cloud, a shell of trillions of icy objects at the furthest reaches of our solar system. "
Seems like a great opportunity to send out some interstellar probes. The star will be trailing lots of ISM, free gas that would help bring a ramjet up to speed, and track till you could curve towards another destination. Likewise, a solar sail probe launched out in front of it by laser could "hitchhike" , and get some deep space ISM , and EM measurements.
Can we think of some other opportunities that this might present ? If we are past the filter by then, then we will already prob have samples of the Oort objects, but looks like they will be delivering then...