Once you have been to the dark side (class 3 28 mph) you can never go back!Changing the front cog will help with the top speed you can comfortably pedal up to without a ridiculously high cadence (at the expense of hill-climbing) but will not help with the motor cutting out at 25 km/hr on a Euro-spec bike, as the speed sensor is on the back wheel.
There are ways to 'cheat' the system - changing wheel size in the computer; using a dongle; moving speed sensor to outside of chainstay & installing magnet on crank; etc.
Personally, I don't feel I need power assist at over 25 km/hr as I want assistance uphill & prefer to pedal under my own steam on the flat (improves range & my own fitness). Gearing is great for my needs on my Reaction Race as this has the XT 11 speed gears. If you want better gear range without > 25 km/hr assist, an upgrade to 11 speed may be worth it for you?