Yep, a few times.
Not saying it's a bad plane but I've found fighting it is very similiar to fighting against the Spitfire Mk IX. Like the Spitfire Mk IX, I either try to keep the fight at high speeds, where the P-38 has the advantage in a high speed turn fight and exploit the weakness of the Ki-84 at high speeds by using right turns. It has also been my experience while the Ki-84 does seem be good at low speeds, it is not as good as the Spitfire in a low speed/stall fight and again you can take advantage of the torque and keep the turns to the right like you do in high speed turn fights. At least to me, it's a little bit more tougher to fight a Spitfire than it is a Ki-84 but that's just me and I'm only going by my experiences in fighting against it.
ack-ack