WARING: This thread is going to quickly degrade into rubbish about the strain "G13".
Anyways. Yes, Scientists could easily make a very potent weed plant. There are only a handful of genes involved in the enzymes needed to make THC. Unregulating them with a virial promoter region would probably turn cannabis into one big sopping pile of resin.
They couldn't change THC, because if they did, it wouldn't be THC anymore.
It would also be possible for them to tranfect the genes needed to product THC into bactaria, and then one could grow them in culture.
My input on g13. The only research I have seen from the government on cannabis and selective breeding/GE is in regards to a ultra low THC strain. It was not called g13. Have a look on overgrow.com for more info. And whatever it was, it wasn't genetically engineered because back in the early 90s GE cost way to much to bother using on cannabis.