On the back of a similar thread on the BAEC board, it may be helpful for those going on miles / tier point runs to know which destinations are preferable for back-to-back / immediate turnarounds.

Thanks to all who have contributed so far. Please add to the below if you have experience of this at any Cathay destinations (edit the wiki below directly if you have recent experience of any destination or you are 100% certain)

Definitions:
- Excellent: you will leave the aircraft and find yourself in the boarding area for departure ("Changi T1, T2, T3 model but not T4, CX's terminal" - no segregation of arriving and departing passengers)
- Good: you have to clear central security and/or immigration before reboarding, but the typical walking + queuing time required and the aircraft turnaround time should be fine
- Possible: you have to clear central security and/or immigration before reboarding, but the typical walking + queuing time required and the aircraft turnaround time make this more risky
- Not possible

Mainland China: Not possible, especially back-to-back (same aircraft) - i.e. XMN, TAO, FUC, CTU, XIY and others
BCN: good (though note it is a 6-hour turnaround so will need to wait)
BOS: possible (though will need to go through immigration and customs and reenter via landside) Although Immigration lines can be atrocious at BOS, there are very few international flights landing at that hour, so it should be reasonable.
CDG: good (though BAEC board does not think so)
CGK: possible (unhelpful transit desk; APEC cardholders may find it more predictable to exit and reenter via landside)
CMB: excellent (though need to print return CMB-HKG boarding pass in advance - see separate thread)
DAD: not possible for U-turns.
EWR: possible (though need to clear immigration and customs and reenter via landside)
HAN: not possible for U-turns.
HKG: good (still need to reclear central security, but efficient. May require high speed walking depending on respective arrival and departure gates. No immigration required)
HND: good (need to clear security; ample time buffer)
IAD: good (though need to clear immigration and customs and reenter via landside)
ICN: good (transfer security available at satellite terminal - no need to take train to main terminal, no immigration)
JFK: possible for CX846/845 and CX888/841 (though need to clear immigration and customs and reenter via landside). Not possible for other flights
JNB: good (though note it is a 6-hour turnaround so will need to wait)
KHH: good
KTM: Possible, but get ISM to notify ground staff so they will assign an escort to take you through all the step. Not Possible without escort
KUL: excellent
LAX: possible for CX880/881 (though need to clear immigration and customs and reenter via landside). Not possible for other flights
LHR: These are 2 hour transit time combos: CX257/238; CX239/256; CX253/254. Walk to gate 3x can be LONG; intermediate transit security area.
Morning arrivals require 6 hrs transit times for 1200ish departures
MAD: good (though note it is a 6-hour turnaround so will need to wait)
MXP: good (though note it is a 6-hour turnaround so will need to wait)
MLE: good (subject to update on separate thread)
MNL: good (best to plan your turnaround on the next flight to be safe. CX906/900 and 918/902/934 depart close to each other to minimise time wastage). No need to clear immigration however your passport will be taken for processing which may take as much as 30-40 minutes.
NRT: possible (need to clear security; tighter turnarounds)
ORD: not possible
PEK: n/a - regular delays mean this is not a viable miles run route (only try to do so if you have a flex ticket)
PVG: n/a - regular delays mean this is not a viable miles run route
SFO: possible for CX872/873 and CX892/893 (though need to clear immigration and customs and reenter via landside). Not possible for other flights
SGN: not possible for U-turns.
SIN: possible, T4-T4 transit may pose some surprise to the staff
SYD: good - transit security but no immigration needed
TPE: good (still need to reclear central security, but efficient and usually empty transit facility)