Tian Huo Zun-ze
What is his true power level after complete recovery and rebirth?
Did his strength increase considerably as to how Yao Chen's did due to increase in Spiritual Strength?
His peak before his death was a level 5 dou zun. It would be difficult to return to this peak however.
edited September 2017
How well you return to your peak after rebirth also depends on the pill used and the quality of the materials (the body and the essence blood). They were rushed for time and Xiao Yan wasn't at the level yet to successfully refine the 7th tier high/peak version of the necessary pill so both settled for the 7th tier middle rank pill. The body was also that of a 2-3 star Dou Zong at best which was all they had to use.
When it eventually comes time to revive Yao Chen, Xiao Yan's ability as an alchemist has grown enough to make a high quality version of the 7th tier high/peak rank pill that rivals a tier 8 pill. The essence blood used (few in quantity high in quality) is better and at a higher level then and with luck that have a stronger body to use.