I got one of the Romanian ones on black Friday. Checked it with boiling water in a sauce pan and it showed just over 100°C so no concerns there. Very easy to fit. Slightly concerned I may have overtightened it as I read you can cause damage to the softer metals on the group thread if you do, but I guess I won't know that until I remove it.
In terms of interpreting what's actually going on, I think it's probably posing more questions than it's answered! A few of my thoughts and observations though: (N.B. my PID is set for a group temperature of 95.1°C)
1) Thermometer shows ~93°C when machine has been idling for >45mins. This can vary by up to 2°C depending on ambient temperature/if heating is on/windows open etc.
2) Cooling flush: Originally I used to do my cooling flush for a set volume of water (~60ml if machine on for 1hr, more if on for longer, based on flashing water and noise). With the thermometer, the temperature will rise during the cooling flush until around 96°C - 98°C depending on how long the machine has been on for. Rather than stopping the cooling flush at a specific temperature I stop it as soon as the thermometer stops increasing. Once the flush stops, the temperature gradually drops back down to ~93°C. While it drops I will prepare the portafilter and then run the shot once it has stabilised. The temperature will usually rise to ~96°C and then settle at ~95.1°C while the shot runs. What's interesting is that my second shot will rise considerably above 96°C at the start with no screen flush, so I now always do a 2 second (longer if more time between shots) screen flush before consecutive shots to keep shot temperature ~95.1°C.
3) Recovery time: my PID ensures boiler pressure is back to 1.2 very quickly. What the thermometer tells me is that the group/water (see item 6 below) temperature drops to ~90°C or lower. Even if it reads less than 93°C it was idling at originally, once the temperature stops dropping the next shot will be at the desired temperature. If I run a shot whilst it is still dropping the shot temperature will be low.
4) I have read about hX machines struggling to do four or more consecutive shots. My 4th shot in a row is still consistently at ~95°C as expected. I haven't made more than 4 in a row.
5) Using the steam wand or hot water tap makes no difference to group temp (as expected).
6) I'm still not entirely sure whether the thermometer shows water temperature me or the actual group temperature? I also think the answer to this question changes depending on whether water is flowing or not.
7) I'm sure there is a whole ton of things you could learn from these temperatures (and I look forward to seeing other people's analyses) but I think the best thing I've learnt (now that I roughly understand my cooling flush and recovery) is actually not to worry about the temperature too much as my machine and PID seem to do a very good job at keeping temperatures where they're meant to be so that people like me don't have to concern themselves with them!