@ElfGunn The most important part of the Fire Phoenix is the effect you get when you feed it a pet food. When fed, it gives all of your troops a 5% damage bonus for each stage it has. So a stage 10 Fire Phoenix is going to give you a 50% damage bonus. This effect lasts for 24 hours, so you can do 2 rounds of the tournament on a single feeding. Royal Restorations will help you upgrade it, but they will not help you evolve it. You must have Phoenix Artifacts for that.
In Beta, they recently put Phoenix artifacts in the Spire as prizes on a temporary basis, replacing the Moonstone Library buildings. There is no official announcement on when this will happen for us in the Live Worlds. If and when this happens for us, you will be able to get Phoenix Artifacts in the Spire of Eternity for a limited time (I think Beta was a 6 week trial).
In addition, the spring event (normally around April) is typically Phoenix themed. Last year the building was the Coldfire Phoenix. However we were give the option to convert Coldfire Phoenix artifacts into Phoenix artifacts in the Magic Academy. The exchange rate was 2 Coldfire artifacts to make 1 Phoenix artifact. Since I missed the original Phoenix event, I did this. There is a pretty good that the upcoming April event will also be Phoenix related, so this opportunity might present itself again in a few months.
Because of the exchange rate my Fire Phoenixes are only partially evolved. But they still make a huge difference. They are absolutely the best building in the game for a combat oriented player. I am eager to finish evolving them so I can get max benefit instead of partial benefit.