Changing weights won't increase top end as you're generally not altering the overall gear ratio.
The engine RPM would need to increase by the percentage speed increase desired, but that is only half the equation as the HP needs to increase also to overcome the parasitic losses as the speed increases.
I've yet to see anyone on this forum alter the transaxle gearing either.
Can the motor spin to a much higher rpm? That's always a possibility, but not certain how well the engine is supported by the aftermarket.
Best bet would be to purchase a different machine or learn to live with the limitations.
2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors