you can, if ya do it right. the enclosure can make up for the lack of power to an extent, as can the type of vehicle, driver positioning, etc.. within reason. I mean you won't get car shaking bass out of an 18" sub with 25 watts or anything but being reasonable, you can compensate for lower power with good system design.
It will still sound fine but it won't be loud. Loud = power and a higher efficiency speaker does a better job at producing sound with the same amount of power.