The speed where one pound of thrust equals one horsepower is 375 mph. That stems from the definition of the horsepower: 33,000 foot pounds of work per minute. That is, one pound of force at 33,000 feet per minute equals one horsepower. When you convert 33,000 fpm to mph (33,000 / 5280 * 60), the result is exactly 375 mph.
You are correct that a pound of thrust at some other speed gives more or less horsepower, proportional to speed.