How tight can I mount a TO-220 to a bus bar?
The part you are trying to get the heat out of is roughly in the centre of the black plastic part and mounted against the lead frame. Distorting the lead frame (the metal part) by overtightening will result in poor heat transfer and could even damage the bonding of the die to the lead frame or the die itself if it bends the soft metal leadframe, even slightly. 

You should not overhang the part that contains the die as shown in the left part of your photo- ideally that's the part you want in intimate contact with the heat sink. The screw is offset because it's inconvenient to put a screw right through the centre of the die but you are really trying to create contact and some pressure between the part under the plastic and the heat sink. Some manufacturers use spring clips that push on the plastic itself. From an Infineon page, here is a part with a huge internal die. Keep in mind that the die is brittle silicon and the lead frame is soft and malleable. 



Vishay Siliconix recommends 15 in-lb (1.7 N-m) torque for their TO-220 packages. There is limited advantage over 10 in-lb, as shown here: 



It's fairly common to use torque screwdrivers and wrenches with power devices, and like anything else they may require regular calibration and testing. Inexpensive ones as used by gunsmiths etc. start at around $50 US.