Will I need an SDS to get through a standard cavity wall?

[rubberduck]

Just have a down angle 2 degrees so any water drains to the outside.

Is this needed if you are going to be flooding the holes with silicone?

 

[Mercenary Keyboard Warrior]

Not really, but silicon can fail and a slight downwards angle is belt and braces.
You absolutely dont want it the other way round