Tar drop rate without thf was lower than expected, around 10-20%. When thf arrived it picked up considerably, closer to 40-50%. It's quite easy to farm for everyone before going after the nm, safest area to pull them is where you fight the nm, in the passage OUTSIDE the cave. There are two Nitro pops in the cave where the ??? is, killing those before each pop should give you any extra tar you need from wipes/dud nm pops.
Actual Soboro drop rate was very good with thf, around 7/9, this is with maybe only one self destruct the whole time. Finally got one!