In theory those stock tires are adequate (barely) for the axles, but that is in a perfect world at perfect inflation. If our tires were perfectly inflated all the time, there would be no need to make TPMS units and for us to constantly be checking air pressures.
In the real world, upgrading tire carrying capacity does give an extra margin of error. A D rated (either 14" or 15") will carry your 7,500 trailer even if not at perfect inflation a lot longer than those barely rated stock style tires. I for one like extra margins of error. Toss in road bumps and stress and that margin of safety of the D rated Endurance comes more into play. IMO the barely meeting capacity of a lot of trailers with those stock style tires "under perfect world conditions" is what lead to the China-bomb name. Since the world is rarely perfect, with little to no margin of error, bang goes the tire.....
I guess it's like trying to tow a "lightweight" 5er with a 1/2 ton truck. You might get away with it and be right at all the limits, but is it really a good idea to roll the dice all the time? The real world has a way of slanting the numbers (and usually not in your favor).
Just the 2 of us in a...
2015 Freedom Express 246RKS
2002 GMC Sierra 1500