15 lbs for a 15x4 is not what I would call light (for reference, my 15x3 supertricks are 8 lbs, but certainly NOT streetable!)
17 or 18 for a 15x8 is not bad. I have a pair of old ansens in 15x8.5 that are almost 23, and my 15x8.5 daisies are 19+.
Yet it looks like they have one 15x8 listed at 23lbs. How?