The manufacturer of the trailer and your mechanic don't have much to lose if you wreck this vehicle by exceeding the towing capacity, and you might even say they have something to gain (less so your mechanic).
Towing capacity is about more than wheelbase and horsepower. It involves the whole drivetrain, frame strength and rigidity, strength and size of the brakes, capacity of the transmission fluid cooler, alternator capacity, and other factors.
Please, whatever you do, [b][i]don't exceed the towing capacity listed in your owner's manual[/b][/i]. You will put yourself, your vehicle, the horses, and others in danger, and if there is any warranty left (like an extended warranty or a 100,000 mile drivetrain warranty), you will void it.