Most Police 135's in this country could go to full fuel and fly for more than 2 hours before reaching MLA. We work on around 200 kg per hour which I believe is standard for the 135 series.
Most of the time we choose to keep the fuel lower than maximum to give us the flexibility to carry out a helipad approach. Some forces have to do this on departure from base anyway.
Certainly I have not heard of a long range tank for the 135, but I guess anything is possible.
Often its not the role equipment that causes weight and C of G issues, its the SLC.