As a bit of a explination of what happens in the transition, this is from quadplane.cpp
// calculate transition rate in degrees per
// millisecond. Assume we want to get to the transition angle
// in half the transition time
so Q_TRANSITION_MS simply sets the target pitch rate for the transition period. Also the throttles are set using
attitude_control->set_throttle_out(motors->get_throttle_hover(), true, 0);
So when transitioning the throttles are just set the hover throttle. So if your seeing dip down when transitioning forward and loose altitude when transitioning back you need more throttle while transitioning. So you must increase q_m_thst_hover.
note that this will chance your gains a bit so it may be worth re-tuning after changing it.