reasons for SI units are obvious: http://edition.cnn.com/TECH/space/9909/30/mars.metric

Examples:

use seconds instead of miliseconds
QTimer::setInterval(int msec)
QTimer::start(int msec)
should be 'double sec'

use radians instead of degrees (or 1/16th of a degree)
QPainter::drawArc, QPainter::drawChord, QPainter::drawPie, QPainterPath::arcTo

base method (without unit in its name) should use SI
QTransform::rotate() // sould use radians
QTransform::rotateRadians // not needed
QTransform::rotateDegrees // additional non SI variants possible