I just updated to 4.5 two days ago. Complete reinstall using Windows uninstall full program, (but NOT deleting the info folders left behind, as these contain all your add-on info,) and reinstall using all individual components downloaded to a single folder. All add-ons still in place, and seem to be available. Only 1 flight so far, testing and allowing system to reload shaders. I did empty the shaders folder in (my) AppData\Local\Lockheed Martin\Prepar3D v4\Shaders. I started my flight at KSFO in Milton's just released FSX Native conversion of his Most Excellent Howard 500, taking a short flight to Travis AFB. I have Megascenery's latest California photoscenery updates, along with ORBX and FS Genesis mesh. I noticed a little stuttering as I got airborne, but that quickly went away. I don't track my frames, just enjoy the flight. I know some stuff goes on in the background on my rig, and it occasionally causes a few stutters, my fault for not turning everything off. But overall, v4.5 looked very good.
I need to run the latest release of AI Traffic Patcher, aittp_v2_0.zip by Jorge Buchelli, available at AvSim.com.