performance upgrade advice please

I've a F23Z4 motor in a mid 2003 Odyssey (previous shape before the new lower"sporty" shape) and would like any advice for upgrading its
performance. I'd like to increase its midrange torque mostly as we find it gear hunting a lot on long hills at around 100klm/hr.It also kicks down too easily at middrange revs, even if it only has 500rpm remaining to the redline. Has anyone good or bad experience with chip upgrades, exhaust systems and other bolt-on mods etc, or should I be looking for more extensive work such as a supercharger?
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Maybe just downshift on the hills?
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