gp performance - internals for Aprilia
Big bore kits - Will give a large increase in torque through the mid range, but at higher RPM smaller hp gains can be expected, unless you are using larger valve sizes and or performance camshafts as the engine may struggle to fill its larger cylinders.
Camshafts - Can give top end gains but often there will be losses and possibly flat spots at other engine speeds especially if used on an otherwise standard motor.
Higher compression ratio - More power throughout the rev range but can cause pre-ignition especially in the midrange. Can work well with high lift or longer duration cams as increased valve overlap can reduce the trapping effects within the cylinder.
Cylinder head work - Most modern sports bike heads are well designed to give a good compromise between maximum torque and horsepower. It is surprisingly easy to ruin a good head by over-enlarging the ports or removing metal from the wrong area. Larger ports increase gas flow and hp whereas smaller ports will increase gas speed and tend to boost low and midrange torque.