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I would like to change the line of thinking here and instead of focusing on top speed in 5th gear, maybe look at engine performance in other gears based on RPM.
Does the bike pull hard through the full RPM range in 2, 3 or 4? Does it feel like the engine falls off a bit in the higher RPM range when going through those gears too? It may be a little less pronounced because the load on the engine is lower, but it should be something you can still feel. The engine should pull nice and strong all the way to at least 7000-7500rpm.
I am still curious if you checked the O-rings on the intake pipe between it and the cylinder head. I would also look over the rubber on the intake pipe itself to make 100% sure there is no hole, tear, or crack that may be causing a vacuum leak. It's not unheard of even on a new bike.
Beyond that, assuming everything on the engine is good, then we need to look at other things that can be causing enough drag to slow the bike down.
Have you checked the rear wheel bearings? The 2017's seemed to have a problem with the factory installed bearings being installed poorly and were locked up and almost impossible to turn. My own bike had this issue.
How much slack is in the chain? If the chain is a bit too tight it will also cause some drag. I have found that to get proper chain tension on my Hawk, with the swing arm at full droop you need around 28mm (A bit more than 1 inch) of free play at the center of the chain - or 15mm with the suspension compressed until the chain is at it's tightest.
Have you checked the rear brake adjustment? It's not uncommon for some of these bikes to come with the rear brake adjustment just a hair too tight. This causes the rear brake to drag, which in the case of the Hawk is quite a lot of braking force since the rear brake is rather overpowered.
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