Mine is mounted flat to the roof and does OK. The Raymarine manual states it has an operating range of 15 deg both up and down from the center of the beam. I've made some wedges and one of these days when I take a break from fishing, I'll install them.
I recently added radar to my 2320...I used a 4 degree wedge along with a 5" Powermount...Both were purchased from BOE Marine. From the picture you can see the wedge keeps the center of the radar antenna horizontal to the surface...The radars vertical beam width is 15 degrees so I don't think the 4 degrees affects performance it just looks better when the radar antenna is horizontal to the surface when the boat is at rest. I added the 5 inch mount to raise the radar antenna above my GPS antenna to prevent interference and damage to the GPS receiver.