Is it permissible to suffice with eight Rak'ats of Tarawih prayer with the Imam, then leave to perform Witr alone, and would one thereby attain the reward of Qiyam al-Layl (standing the night in prayer) with the Imam?

Tarawih prayer consists of twenty rak'ahs according to most scholars, and this is what Umar ibn al-Khattab gathered the Companions upon. As for Witr, its maximum is eleven rak'ahs and its minimum is one rak'ah. It is recommended to stay with the Imam until he finishes so that the worshipper may be credited with the reward of spending the night in prayer, based on the Prophet's (peace be upon him) saying: "Whoever stands with the Imam until he finishes, it will be written for him that he spent the night in prayer."