There are a few general considerations that need to made for a good quadplane design.
The reason to tilt the hover motors forwards, and why we do that and have used it successfully, is that any positive increase in the angle of attack of the main wing will produce stabilizing lift in hover, especially in wind, whilst using the forward motor assist for position hold. Having a negative wing AoA, which is typical for how normal quads work to fly forwards, adds considerable extra load on the quad motors that can cause them to fail, overheat and at a minimum use excessive power simply to overcome the wings negative lift when pitched forward in quad forward flight.
Accordingly, to overcome this, the quad motor AoA should be a few degrees more than the normall AoA of the wings in forward flight. Typically most wings by design stall at the tip and not the root of the wing causing the aircraft to roll in the direction of the tip stall. Using quad assist means that the main wing cannot stall and roll towards a wing tip, because the quad motors provide attitude control (roll) beyond what the wings surfaces are capable of at low airspeeds. This means that the wing can operate at much higher AoA than is usually the case without quad motors, and the added AoA results in much better low forward airspeed wing lift.
This applies to flying in wind as well whilst the airframe is stationary to the ground position, to the point that with forward motor assist on for position hold, increasing wind reduces the quad motor power requirement to position hold significantly, to the point (about 14m/s and above for the Mozzie) the quad motors turn off and the airframe remains stationary relative to the ground. (I've flown it in 80kmh winds). The reason for this is fairly obvious in that wings are subject to airspeed not ground speed, and having a positive AoA at all times allows them to always contribute lift.
A quadplane should never be flown forwards using the quad motors alone, like a normal wingless quad. It should always use the forward motor for forward flight instead.
The only real issues with tilting the quad motors forwards is their affect on forward flight performance, and this relies heavily on the quad propulsion layout and design. Using larger quad props with a forward tilt angle will likely increase the forward cross section of the aircraft, and can result in significant unwanted auto-rotation of the quad motors and props, which in turn will generate unwanted lift and drag in forward flight. The wings are much more efficient at producing lift in forward flight and should be allowed to do so. There are also some secondary effects of having larger props, and spinning props, in that they can cause significant disruption of laminar airflow across the wings which reduces forward efficiency as well. Hence our desire to reduce the prop and motor size to the bare minimum to improve forward flight.
If using larger props that want to spin on forward flight, I'd strongly recommend using the ESC brake function to stop them. Using larger quad props not only interferes with the aerodynamics it can also cause side effects (voltage spikes etc) on the drivetrain depending on how good your ESC's are or how they are configured. There are only a few ESC that will use a encoder to stop them in the right position for forward flight, so the best way is to simply not use large props.