Screaming engine is the road most travelled, so there is plenty of advice here on how to do it right. Higher compression will get you more power everywhere for sure, although it adds more stress on rod crank and bearings and you'll need to run premium fuel. Porting will definitely improve flow. If you go the tb cam route like everyone else, you'll exchange low rpm torque for high rpm horsepower. That's a trade everyone here is happy to make.Trying to figure out how to get the best of both worlds (low response and high rpm performance). The only way that I see is if I get head port service, and flat top piston. I'm trying to make this 125cc scream.
For how I use my Grom, I like an engine that pulls strong from 3,500 rpm and does 26 mph in 4th gear without lugging. I almost never pin the throttle, the engine rarely goes over 7,000 rpms unless I'm doing 55, and it does that easy without full throttle. The bike has absolutely NO problem keeping up with the vehicles around it without needing to spool up the engine. It's a joy to ride.
The Grom is not a high performance motorcycle... unless you want to make it into one.