Howdy Chaz;
Rear wheel horse Power is the power that meets the road. It is also known as net Horsepower. It takes all parasitic power drains into consideration. It is identified on a chassis dyno. The old way was Gross HP. It was usually taken on an engine dyno, sans alternator, trans, and Air cleaner.
Curb weight is the weight the factory assigned to a vehicle combination for basic shipping and (In some states) registration. Actual weight is/was frequently different and usually more. Curb weight does not have anything to do with RWHP.
Weight will have an effect on performance. Less weight will have a positive effect on both acceleration, economy and handling. Drag racers figure a loss of 100 pounds is worth a 10th of a second in et, road racers not only try to be light weight, but they get obsessive about unsprung weight and corner weight.
While you can identify a lbs/HP for a vehicle using factory HP and weight statistics, that goes out the window once you start to modify, or change vehicle content and components. The only true rule is that "Less is better", so long as you maintain safety.
My '65 Ranchero, with a much modified 250, an SROD 4 speed comes off the transfer station scale at 2,380. That was before the paint job, Rhino bed coating, Ultra Lazer wheels and new carpet. It's time to weigh again.
Adios, David
