I think what he's saying is that if you change your sprockets to a lower ratio you can get a lot better acceleration, which is totally true and worth looking into, but it will kill your top end. If your happy with the way the gearing is set up now the only way to get more oomph is going to be via engine power.
I have a subaru engines as well the only mods i have done to it is bore the carb out some and i also bored out the block ports and now im running a 3.5 gear ratio with 11 inch tires and it out runs my 10hp OHV B&S on the topend