Generally speaking, a quality overhaul (aka shop visit) can fully restore an engine to new performance level. But it costs plenty, and if a lot of time-limited (or cycle-limited) parts are involved, it may be smarter to buy a new engine which will have all the product improvements.

The retired engine can be stripped of usable parts; and the exotic alloys in scrapped parts have plenty of scrap value.

Gearbox torque limits are also sometimes limited by mounting considerations, ie the load capability of the gearbox mounting pads and struts and/or the nacelle structure. The gearbox overload safety factor is often way above these structural considerations. As for EGT/TIT or any other temp monitoring system, I don't think it really matters, as the manufacturers limitations will be based upon airflow models for that design.

If you are speaking about the TPE an HSI will cost about $ 50K, an OH about $100K. An used serviceable TPE-11 is about $ 150-200k.
If you get it from an good (mabe not cheap:E) OH-shop you will have about 30°C EGT margin for your technicians to compensate for power loss over the time. This normally takes you easy to the next shop visit.