99% of my experience is with bigger car engines (been around drag racing since I was a kid and building v8 chevys since I was 12) and I'm not Xlint89 but most engines you will only notice any benefit from opening up the air flow in and out in the upper RPMs. That said yes if the intake or exhaust is grossly undersized then yeah you will notice it in all RPM ranges but on most gy6s its not the case.
The reasoning behind this is if you think of the engine as an air pump at idle it doesn't require near as much air as it does at say 8k. I can get into the black magic of restricting the air in the exhaust more commonly called back pressure and yes that does move the power band down lower but we were talking about opening things up.
Again from experience you don't notice any real gains with bottom end unless you do something mechanical. Could be cam, slidlers/rollers, gearing, or so on.
HP and torque is a balance scale. Its easy to add to one side yet it almost always removes from the other. It's much harder to add to both and get an even result. That doesn't matter if its a 3hp briggs or a 575 chevy.
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