Trimming the elevators fully up would make you prone to overinputs in the pitch axis - and the Ki-84, is not a very stable plane. Well, it's stable enough to use the minimal stall limiter settings in the turn radius tests, but it still has some nasty snap rolling characteristics when the plane enters uncoordinated flight - which is, however, neutralized in most cases, thanks to the excellent flaps.
Kinda awkward, since you're a much better pilot than me wetrat, but I would recommend a neutral elev. setting during combat, trimming up only when high-speed maneuvering is involved.
As for combatting Spitfires.. I find horizontal turns much more effective than vertical turns. Turning in the horizontal plane is a downhill fight. Massive amount of E dumped and none gained. And in this kind of fight, the Ki84 can drop speed fast enough to pull down flaps early in the engagement - once the flaps are out, its no contest. While the Spit5 has a little bit smaller turn radius than the Ki-84, once the flaps are out the enhanced stability of the Ki-84 makes it the winner. Every Spitfire in the game between Mk.5 to Mk.16 is outright outturned pretty easily once the flaps are down.
The problem is, when vertical turns are involved, the downwards part of the loops/wingovers often increase speed in the Ki-84 that the flaps are retracted... and this makes the Ki-84 lag behind the Spit a little bit when going downwards.. not to mention that when the flaps retract, the 'twitchy' nature of the unstable Ki-84 returns... Another problem is when the vertical loop happens at too low speeds, the Ki-84 shows a tendency to fall under a lethal inverted flat spin.. It's very small bit of advantage to the Spitfire, but some excellent Spit pilots can use that to their advantage.
So in my case, I try to force the fight into the pure horizontal plane - where both planes run out of alternatives and must(!) finish the fight no matter what. Once the flaps pop out, within two~three circles, the Ki-84 can get behind the Spitfire decisively, managing a very good gun solution.