"Bites tongue at previous work to resolve fault"
Best thing you can do now is go back to AT and get them to reflash it with its original software then take it to somewhere that can actually fix it.
It wont matter that it has no dpf for just now, you need someone that can look at the live data, theres plenty of it on that to make an informed decision as to where the fault may be and where to test next. Any garage that replaces parts on the basis of a fault code without checking the integrity of the part are amaturish at best.
Your car originally had a boost circuit high code- first off was this even a plausible figure? In your case it probably was because they renewed the sensor and got the same result (see where checking plausability comes in)
Next a dpf with a 60% soot loading was removed and car remapped?
Since 60% is a quite normal & safe soot load, if they were suspecting the dpf to be blocked they should have checked the plausability of the pressure differential sensor, this would have confirmed if the dpf was actually worse (or indeed better) than assumed. Did anyone even carry out a forced regen?
Now that it has been "remapped" (generalisation for someone rightly or wrongly modifying the software in your ecu) you cannot rely on live data or fault codes as they may have been modified!
If AT put it back to standard you can verify the existance of any faults or indeed if its the remap that is at fault. Then when all is well get AT to remap it again.
Previously owned Alfas - Alfasuds (first car was a Ti Cloverleaf) 33's, 75's, GTV's (Bertone & 916), Giulietta (1983), 164's, 145's, 155's, 156's, 147's, now Brera!