Been building and testing and DIY my groms since 2014 when they were first sold in the USA, from stock 125cc to slowly adding and testing parts for the Grom.ARacer seems to be a nice buy if your going to continue to mod the grom and bbk it in the future but if your going to stay with the basics the re flashed ECU is the way to go
ECU's, I've tested Finbro ECU's, Yuminashi ECU's, piggy back PC5-WB02, Aracer mini 2+ and now mini-5. And the one that works and walks away from the contenders if your map is built correctly is the Aracer product for EZ of use, EZ of mapping, EZ of use with out a computer hook up and if you want precise mapping that the use of a computer is the best bet using a dyno machine.
You want to go fast, these things work for the grom
1. Airbox mod and matching exhaust system
2. Ported head and match porting
3. correct throttle body and port and polished intake pipe with port matching to the intake port of the head.
4. correct F/I and cam setup for the type of engine you are building.
5. Kitaco clutch side cover, best first replacement item for any grom or monkey bike.
6.Sex Machine clutch kit and CNC clutch spring plate.
7.HD crank and rod
8. Correct compression ring and second ring gapping, "a must"
9. modified oil pump if you are going to be doing a lot of wheelies
10. Crank case venting system, Takegawa oil catch can system or system like it.
11. GromFatherZ adjustable cam chain tensioner
12 Koso cam chain arm kit
13. external oil cooler system
14. monitoring system for the A/F, oil, and head/cylinder temp monitoring.
Good Luck building.