We present the photometry of 16 91T/99aa-like Type Ia Supernovae (SNe Ia) observed by the Las Cumbres Observatory. We also use an additional set of 21 91T/99aa-like SNe Ia and 87 normal SNe Ia from the literature for an analysis of the standardizability of the luminosity of 91T/99aa-like SNe. We find that 91T/99aa-like SNe are 0.2 mag brighter than normal SNe Ia, even when fully corrected by the light-curve shapes and colors. The weighted rms of the 91T/99aa-like SNe (with
zCMB > 0.01) Hubble residuals is 0.25 0.03 mag, suggesting that 91T/99aa-like SNe are also excellent relative distance indicators to 12%. We compare the Hubble residuals with the pseudo-equivalent width (pEW) of Si ii ii 6355) increases, the Hubble residual increases when ii 6355) < 55.6 . However, the Hubble residual stays constant beyond this. Given that 91T/99aa-like SNe possess shallower Si iilines than normal SNe Ia, the linear correlation at ii 6355) < 55.6 can account for the overall discrepancy of Hubble residuals derived from the two subgroups. Such a systematic effect needs to be taken into account when using SNe Ia to measure luminosity distances.