This sounds like a question for the people at Speedbox3B. Their product is suppose to disable the speed limiter.
Just for your information Brushless motors have KV ratings based on the way their coils are wound. This determines how many RPM the motors will turn per volt applied to the motor. In your case it could be the reason you top speed is 28. But if that speed is a hard cutoff I would suspect an electronic speed limiter.
Does your display have a wattage display or some sort? If it does watch the display as you near the 28 mph limit see if the power drops off as you reach that limit. This would indicate your speed is still being electronically limited probably by a speed sensor. If so the only way to get around that is to reprogram the controller somehow in most cases not doable.