My personal opinion is they are a waste of time and money and can be worse not better.
A bit like the Crypton tuning of the old days.
You can't make a silk purse from a sow's ear, as they say
from my own personel view in fitting a synergy box it has greatly improved my td4
with the box 40 mpg on a run
just had it updated and rechipped and waiting for it to come back
the flat spot leading up to 2000 rpm was gone
far more responsive
however ive got mine set up for max mpg and a bit of extra torque if i need it
it will have a switch on the dashboard, set up for eco and power
i know its a 4 x 4 and treat it accordingly
but of course thats my own personel view and dont know what everyone else thinks if theyve fitted a synergy box
however if u have faults on ur car on fitting one will as u rightly say make the situation worse
crypton days, now ur going back and the small glowing spark plug , lol
think weve moved on since then