"Hau rein".

"Cool, mach das!"

"Hau rein" is pretty good, I'd say. Yet, I would never probably simply say: "hau rein!" as the answer to you question, I would always say s.th. like "cool" or "klar, mach das!" Otherwise, it depends too much on how you say it to really make it encouraging... errrh, I understand "Go for it" to be encouraging and not cynical. ;-)