According to Sheikh al-Albani:
The Prophet (sallallahu aleihi wa sallam) did not pray more than eleven rak'at for qiyam (thirteen if we count the two short introductory rak'at).
'Umar (r.a.) commanded Ubayy Bin Ka'b (r.a.) and Tamim ad-Dari (r.a.) to lead the people in tarawih with eleven rak'at, in accordance with the authentic Sunnah.
All reports that the people at the time of 'Umar (r.a.), or any of the noble sahabah, prayed twenty rak'at are unauthentic and contradict the truly established reports. Similarly, all claims that the sahabah (r.a.) have made an ijma' (consensus) to pray twenty rak'at are baseless
Even if a weak report were considered authentic by some people, one should still follow the well established authentic report, because it agrees with the number established in the Sunnah. One may not desert the Sunnah for anybody's understanding or practice.
With our certitude that the Prophet's (saaws) consistent practice is the best, it becomes obvious that one should adhere to the number established in the Sunnah, and refrain from adding to it. This has been the position of many 'ulama.
And even if we concede to the often misquoted and misunderstood statement that, "There are good innovations," we must adhere to the consensus among the scholars that following Sunnah is better that any innovation. 'Abdullah Bin Mas'ud (r.a.) said: "Doing little according to the Sunnah is better than doing much in way of bid'ah." [Authentic report recorded by ad-Darimi, al-Bayhaqi, and al-Hakim. The latter verified it to be authentic - to which ath-Thahabi agreed.]