Is it permissible for a worshiper to say the opening Takbir (Takbirat al-Ihram) while sitting with the Imam, then stand up after the Imam's recitation, especially in Tarawih prayers where the Imam prolongs the recitation?

It is permissible to sit during a supererogatory prayer (nafilah) without an excuse, and the reward for one who prays sitting is half the reward of one who prays standing. However, if there is an excuse, then he receives the full reward of one who prays standing.

As for obligatory prayers (fard), it is not valid to perform them while sitting if one is able to stand; rather, the one who does so commits a sin. If one prays the obligatory prayer sitting due to an excuse, then he receives the full reward of one who prays standing.

Based on this, one can begin a supererogatory prayer standing and then sit, or begin sitting. If the worshipper feels energetic and stands after having sat, that is better, just as the Prophet, peace and blessings be upon him, used to do.