I’ve gone and tested this in the scenario editor. Given the ratios I have above (8 JPKs and 11 Slingers vs 5 Ronin), you will always win. This is without the community plaza.
All of your JPKs willl die, but you should still have 90%+ of your Slingers left. That means you are trading very cost effectively. And again, this is without the community plaza.
This is consistent if you scaled up to 40JPKs, 55 Slingers vs 25 Ronin.
This is consistent for fully upgraded Swede Ronins (3 cards + age 5 Politician + Arsenal) vs fully upgraded JPKs (4 cards + Legendary) with non-carded fully upgraded slingers (Legendary without the temple card).
You’ll also be able to re-mass significantly quicker than a Saloon unit.
JPKs only vs Ronin is a loss however. You’ll only kill ~50% of the Ronin. You need the ranged support.
Surprisingly, there is a threshold where Legendary Slingers start winning on their own. 28 vs 5 Ronin will lose (1 Ronin left with ~200hp), but 142 vs 25 Ronin will win (with 59 Slingers remaining).
RE:Siege - not much you can do there unless you want to do the building HP dance. Might be useful with Slingers behind?