When i bought my 120 it was prone to issuing black smoke on acceleration, it also sounded like it was 'pinking' under acceleration.
On examination i found a Tunit box was connected in place, once removed the black smoke stopped instantly and that pinking sound vanished with it.
Can't say as i noticed any loss of power removing the tunit box nor any real difference in fuel usage but it was only in place for a matter of weeks before i found the thing, its now sitting in the cupboard above me, unless SWMBO has tidied up in which case it will never be seen again, i won't refit it because as the Aussie Prado youtube warns us overfuelling is not good for these engines and can lead to cracked pistons, thankfully there appears to be no damage from the thing which should have shown up over the years since if there was.
The previous owner was a caravanner and had a new towbar fitted at great expense at Toyota couple of years prior to our ownership, i suspect he bought the unit for some reason.
What has improved power and running is cleaning the egr and, ahem, removing that pesky throttle butterfly, when breathing clean fresh compressed air these vehicles are plenty fast enough.