So why the new nose?
McLaren were at odd's with the other teams this year who had opted to run much closer to the highest allowable surface area's which has resulted in the ugly 'step noses'. It's not only the nose on the McLaren though that was set much lower. To allow for the gradual curve we have been used to in F1 McLaren also have a lower bulkhead than most of it's oposition. Their design centre's around having the driver at a lower level thus lowering the overall CoG, this also means having the suspension geometry lower and so no need for the higher bulkhead. (A higher bulkhead would have meant raising the driving position for better field of view)
For a few years now McLaren have run with the 'snowplough' under the nosecone as their turning vane(s) the snowplough also acts as a vortice generator creating energised flow toward the front edge of the floor. The original iteration of the McLaren nose also ran with the 'snowplough' (Jerez below) with the FOM camera's outbound of the wing pylons.
Toward the end of the Barcelona test (Below) McLaren tested with the FOM camera's moved into the central section of the wing and removed the snowplough in favour of some much more neutral turning vanes. My best guess for this solution is that although the snowplough is a very effective element (acting as an additional wing) with the loss of EBD having the vortices it creates interacting with the front of the floor was bad for 'aero' business. Things need to be balanced and so there is no point having something generate downforce in a certain area if you can't attain the same net result further down the chain.
Without the use of EBD this year the aim is to gain back as much rear end downforce as possible and this is the biggest reason as to why the teams went with the 'step nose' designs in the first place. Having the nose in the higher configuration will allow more air to flow under the car thus speeding up the diffuser's effectiveness. McLaren were trying to play cat and mouse with this flow area and rob some airflow to generate more downforce atop of the floor, it's a balancing act of 1 flow vs the other.
The new nose is a compromise as McLaren don't have the bulkhead height to warrant the step, this also means they don't have to have such an agressive ramp styling from the bulkhead down to the nose tip. Their nose has more scope to fit within the height regulations however due to the bulkhead dimensions it does mean a sharper under nose gradient back to the bulkhead. I don't see this as a bad thing as it should redirect the flow lower as the air detatches from the underside of the nose pushing the air below it to do the same. I think we may also see a different set of turning vanes appear on the car soon in order to better utlise this flow or maybe a reimagined snowplough.