Hi David,
I fly a twin turbo - TSIO-520-WB which is similar to your TSIO-550 K in some ways (but not all). This is my first turbo plane and I’ve spent a decent amount of time reading about it’s operation. This is just the stuff I’ve learned so far. Take it for what it’s worth!
The first thing I’d recommend is reading the Continental manual for your motor. The aircraft manufacturer has some er hem, “leeway” in how they choose you operate the motor. This may be on the high end of OK in order for the plane to look its most capable but may not be the best for the motor. For example for my Baron, the POH says go to red line on the TIT (899C, 1650F) when leaning which isn’t what everyone says is a good idea. I’m not saying that Cirrus has done this, I’m just saying read all the valid documents and form your opinion within the bounds of the limits.
The engine manual may give you a range of other options as well as some interesting tables on fuel consumption and power that can help you see if you are getting your book values. I recently went through this exercise and concluded that my fuel servos needed to be serviced. Lo and behold they were not in great shape! This is unlikely to be your problem but my point is only that you should know your engine and its performance parameters so that when things “get weird” you know.
To answer your first question about exceeding: the TIT is the downstream product of all of the firing cylinders. Chuck Cook recently told us at a clinic that leaning to peak TIT is sort of like moving the heat problem around. Yes, fuel economy improves but wear increases in other parts of the engine. In a richer mixture the cylinders may capture and dissipate the combustion heat more efficiently versus some of the charge making its way further down the exhaust system.
The risk as I understand it for exceeding peak TIT is the potential of fatigue in the vanes of the impeller as well as the seal that keeps the oil flowing through the turbine wheel for lubrication and heat removal. A failure of the seal will usually leak oil, potentially into the exhaust system.
Second, remember that your exhaust system is also exposed to those higher temperatures. This can cause fatigue for your exhaust components as well. Broken exhaust pipes are not great.
Your question did prompt me to look for something that does not seem to exist. There appears to be no document that mandates any sort of inspection for TIT exceedance from Continental. There is one specifically for RPM exceedance so I would suspect if this was truly a safety issue a similar document would exist.
That said, broken turbos or holes in your exhaust are not things we should invite into our flight plan.
On leaning - remember that your engine automatically compensates for altitude.
We lean to TIT - so your normal maximum TIT is 1750F with the actual max at 1800F. So there isn’t really much of an issue here from what I can tell - I think you are doing everything right.
General rules are:
#1 Always climb full rich even for enroute altitude changes. The risk is that the excess heat of full power operation plus a reduced mixtures reduces your detonation margin to the point where detonation does occur. I know this seems like extra work enroute but on the off chance that you forget to enrichen the mixture and don’t notice you could run beyond peak permitted TIT while doing an enroute climb. Also in climb pay attention to your CHTs. In the summer I have high CHTs fairly quickly after departure. I manage them with airspeed and to a less extent, power.
#2 If you reduce power when LOP you are actually increasing the mixture which is the same as enriching back towards peak, the system will get hotter. Reducing power ROP is the opposite, the system will get cooler. However, all of the values you mentioned are fine. So as long as you keep it below the top of the green arc for TIT.
#3 Since I’ve been flying ROP something I’ve been playing with is leaning on descent to keep the engine warmer which the POH alludes to. I asked Chuck about this and he seemed to think that it doesn’t matter.
The POH provides a good guide for cruise ops:
Target fuel flow is determined using a calculated engine air flow based on
Engine Speed, Manifold Air Temperature and Manifold Air Pressure and
indicates a fuel flow that will give the approximate air-to-fuel ratio for best
economy operation. Alternatively, the mixture can be set by finding a fuel
flow that provides peak TIT and then leaning until TIT is 50-75 °F less than
its peak value.
Target Fuel Flow is advisory only. This indicator or the Peak leaning
method will provide an initial lean point only. As this setting is dependent
on ambient air temperatures, it may not ensure sufficient cylinder cooling.
If any CHT are greater than 420 °F, lean the mixture to maintain cylinders
below 420 °F. As an approximation, a 0.5 GPH reduction in fuel flow will
reduce CHT by 15 °F
In the end this is just an exercise in heat management. So pick flows and % Power settings that keep your CHTs in a good place and keep the TIT in the green Arc.
What a “good” cylinder temperature is, I don’t know… I think that is another discussion 
Also knowing the failure modes for the turbo is super important.
