The engine's port and valve sizes, cam events, manifold design etc. were all developed to produce efficient power in the RPM range you want.
The engine's VE at those speeds is already over 90% (and drops off sharply after 3,000), so the most effective method of increasing torque are:
1, displacement
2. static CR
Both expensive.
Most conventional methods (porting, cam event duration, big valves, rocker arm ratios) just move the torque curve ahead to higher RPM.
