like everyone her has said, a Honda 4x4 atv is a low HP machine, geared more for work
there at about 30 hp, as to most all like sized atv's from other brands, 50+ hp range
and there is very Little you can do to make a honda atv make more hp, the amount of money it would cost to gain a few hp, is silly and honestly just a waste of money IMO
a gear reduction will get you more power to TURN the tires, as when you went to 28 inch tires over stock 25's
you LOST power in the process, taller heavier tires rob power, and alter gearing!
so a GR will correct the gearing, but your still loosing top end speeds due to the gear reduction, and its NOT making any more HP, and you still have the heavier tires to take power away!
smaller tires will lower stock gearing and make better use of the power, but NO one wants a atv on small tires it seems anymore!
your best option here is, honestly sell the atv and buy a more sporty atv, it will be money better spent than TRYING to make this honda what you want it to be!
this si why folks need to do there homework before buying, so your not wasting money!
best of luck to you!