hmmmm i dont get the logic also eh. but you can compute it based on the car manufacturer's published figures, e.g. claimed crank hp and vehicle weight. you can compute it yourself. wala naman sigurong range na excellent or average hp-weight ratio.
kaniya-kaniyang benchmarking/preference lang siguro. also, mas dapat ata tingnan ang torque kaysa hp since this is the biggest factor for acceleration. pang high-end high speed na ang high hp figures IMO.
standard disclaimer - power/weight is not the only factor to consider when determining the sportiness of a car...acceleration, steering response, gear ratios, braking, grip, etc. are all important things to consider as well. basically all the things you can research in a magazine and more importantly, discover for yourself on a test drive.
that said, i would go with the idea of a poster above me and find a car that is considered to have excellent/ideal power to weight ratios and see if you can find a car that comes close. for me, my ideal car would be the Lotus Elise, which imo is the closest you can get to a street-legal race car (power-weight ratio is 220hp/t for the US spec)
power-to-weight ratio is just a very rough guide to performance. for example, if objectively, the P/W ratio seems acceptable, it can still be, subjectively, a sloth. maybe the engine revs so damn slow. maybe the tranny isn't so good. maybe the power delivery is peaky and torque is lacking at low rpm. maybe the gearing is extra-tall (or extra-short).
that said, siguro mga 20 lbs. per hp okay na hehe.
take a look at the elise, kahit 1.8 liter toyota engine lang sya, it is considered a high end sports car, because of its very low weight, and other things i'm sure.