I would find the cause of the loss of power before you think about an upgrade, turbos tend to either work or not so the problem probably lays elswhere, injectors as mentioned, MAF not working correctly or also the inlet manifold, the 2.4 suffers the same leaking swirl valve issue as the early 1.9 jtdm's.
If you decide to replace the turbo for a standard spec unit it worth concidering just changing the center of it if the vnt is ok, a chra is a lot less expensive than a complete turbo.
couldn't agree more,
I assumed you had established it was the turbo and TBH at 144k miles I would even expect it...
black smoke on acceleration can only really be one thing , unburnt fuel, so it is down to a number of usual suspects.
1) boost leak either from top or bottom hose, sometimes it can be the intercooler itself.
2) Vac leak, really common that the little vac hose to the VNT actuator rod has a split meaning the VNT is not fully actuating.
3) sticky vanes in the turbo, there are cleaners such as innotec in tank and actual turbo cleaners that you can try but IMO its a stay of execution, it depends on how long you intend to keep the car but I would just refurb the turbo.
4) MAF,MAP, or vnt actuator solenoid is faulty or on the way out.
5) sticky/stuck EGR
Re replacing the Chra in the turbo, well the turbo has to come off and go back on, there is £300 right there. then the chra will cost you £150-200 to buy and then if you are sensible you will want to get the VNT actuator re-calibrated as there is no guarantee it will work as it should with the new chra...
all things considered you are better off just taking the turbo off and having a full refurb somewhere reputable (I can highly recommend turbo dynamics) who will do all of the work in house for around £400..you will have a guaranteed result and the turbo will come with a full warranty.
good you have had the clutch done so no other expense..
but as stated by Jon156veloce track the current problem down first.
I would start with Vac hose myself on this as it sounds very much like those symptoms plus the hose itself is pennies to buy, I think its 3mm internal vac hose.
go with turbo hoses next and then start looking at EGR then sensors and actuator solenoid if no joy.