If you mean a wedge off the front of the box (above the cab and not connected to it), then I would assume that would lead to a lot of wind resistance if not done correctly. It's an issue that overland and expedition guys face when building their campers. And if those DIY guys notice an issue, with their blatant disregard for FE, then I can only assume it most be bad
I would think raising the roof of the cab idea would be better. Then again, I might be misunderstanding what you are referring to by "wedge".
Also, I can't tell in the photo... But if the cab isn't attached to the box, and you create this high cab extension that is molded the length of the box, you are in for some serious issues due to body/frame flex. You might be even if the cab is already attached. I know this is a consideration/issue when designing campers/RV's on medium and light duty trucks. I'm not an expert on ANY of this though.
~C