First of all, please allow me to clarify one thing: there are no performance issues observed so far in GD's engine; it is just notorious for being fuel-hungry. In fact, a fellow member, shedatali, has himself beaten a CG125 with his GD 110: Race with a CG125.
Sprinter's/Raider's engine is OHV, whereas GD's engine is OHC with a longer stroke and thinner bore. I do not know the effect of stroke and bore, but I remember some PW member mentioned that a longer stroke gives a better torque. However, this comes at the cost of more fuel burning.
Someone please explain what advantages do OHC/OHV engines have over each other. Also, why is an OHC engine doing poorly in the fuel consumption department?