No personal experience, however I do have an opinion!
You could use your 300WSM with Barnes TSX or TTSX, Nosler E-Tips, Hornady GMX and have no problems with a neck shot on giraffe. Check Africa Hunting.com. Seems lighter rifles are used quite regularly on giraffe for neck shots.
Otherwise, I vote for the 338WM with a mono bullet listed above or the Swift A-frame or a Woodleigh.
