The 7700XT is much faster in compute but that's perhaps not as important as it having somewhat greater memory bandwidth (430 vs 380GB/s). That is going to help.
And as you say there's also the Wave Matrix Multiply Accumulate (WMMA) support which may give a nice boost too.
The RDNA3 part would seem to be the way to go unless pricing is very different.
No reason to assume so. The largest players in AI/cloud, Google and Microsoft, are firmly on track to become carbon neutral and make significant investments in renewable energy.
Using energy isn't the same thing as creating emissions -- it depends on your source.
Machine learning also has the ability to streamline many energy intensive operations. One recent example is DeepMind generating an accurate 10-day weather forecast in under a minute which used to take hours of computation.
Or significantly speeding up drug discovery and materials research cutting out lengthy rounds of experimentation.
There's not much in the way of comparative ML benchmarks out there so hard to say really. There is this test which shows the 7700XT being 300-426% faster than the 6700XT in Stable Diffusion (depending on image size) which is a significant difference.