I have the Core RX 7900 XTX Reference waterblock.
First issue: Instructions in the box do not tell you to remove the plastic spacers prior to installing the PCB. The updated instructions on the alphacool web site have this added as the first step, so there must have been a misprint in the instructions that went out in the boxes. Before I realized what was happening, the PCB had a VERY severe bend since I hadn't removed the spacers but I was trying to tighten the screws around the core. It seems I caught it in time to save the PCB, but as someone who is usually very observant and careful when doing this kind of work, this was really disheartening to see the instructions in the box lead me to bend my card so badly.
I put this card (and my loop) together twice. Once before reading this thread and once after. My temps are a little better the second time in some situations, but not all. There is still a tendency toward a ~ 30 degree delta under load, between GPU temp and GPU hot spot in HWiNFO. For some reason AMD Adrenaline doesn't usually report as high of a delta, but sometimes it does. This is with the card undervolted to 94%, stock power and clocks. At idle the delta is only about 6 degrees.
The first time I just tried my best to tighten the screws based off of "2-finger" guidance. The second time I used EK's 0.6 nm torque screwdriver on all screws (core and backplate) and I think this did give me a better result, but not by much.
I believe these are phillips #1 screws: I mistook them at first for JIS and stripped one out

It would be nice if I had thought to look up what an "M2" screw is to realize it has a phillips head. The next time I take this card apart I will be using a screw extractor and will need to have a replacement on hand.
My next step is to add more radiators, because I think just having a single 280 mm rad is hindering my true longer testing on video renders and gaming that I have been doing. I am going to add back in my 120 mm and 240 mm rads for a total of 640 mm and then i will see if the temperature delta between GPU and GPU hot spot is more manageable or not. I think what may be happening is it gets unmanageable when there isn't enough radiator to remove the heat, because in the first few minutes it does seem to be under control (hot spot in 80s and 90s). But it'll go above 100 if a demanding workload is running for too long and then I have to underclock for it to be sustainable. I do video renders for hours on end that are very GPU-intensive and currently with this waterblock I must underclock my card to 80 or 90%, but hopefully more radiators will change that.