With the f150 there are just too many footnotes that go along with the max ratings. It may pass the SAE j2807 tests but that's a very controlled environment that the average consumer can't duplicate in the real world. If you get a well balanced trailer and keep the CG in the proper location when loaded add a properly setup WD hitch a 8k trailer is within reason. You can't deny physics though that says a heavier TV will be less likely to become unstable than a light one all other things being equal. A f150 will be about 2000lbs lighter than a f250 gasser, that adds quite a margin of safety for the f250 when pulling a TT.