I seen sumptin like this in a containment mod skinned to be bg. my only thought would be to possibly open the dds and put the same image in there 4 times and try that so whgen it strethes it not so bad.
Your using a low resolution image and stretching it accross the entire bsp floor...Theshader was not meant to be used like that...Modify the original one and it may help...
There's no way to re do the bsp to properly suit your needs the way your asking.