There's a decent chance this had been done before but here goes anyway:
1.) Bendalloy time bubbles make time pass more quickly for the inhabitants of the bubble.
2.) Bendalloy can compress 2 min into 15 seconds or 8x speed (I've heard it argued sliders can create bubbles going greater than 1000x speed.
3.) When multiple bubbles occur in the same place, their effects are multiplied. Source below:
4.) A Bendalloy savant could anchor the time bubble to themselves. Source:
5.) When an object leaves a time bubble, it's kinetic energy is stripped to that of the speed it would be going if it had never been in the time bubble. Source:
FTL using allomancy should be possible using only Bendalloy. Given that the average human can can typically run a mile in 7-10 minutes, and an allomancer has other advantages such as Pewter, I'll be assuming for simplicity purposes that a mistborn can run a mile in five minutes. Therefore, in 15 seconds a mistborn could run roughly 40% of a mile (2112 ft) in 2 minutes, or 15 seconds outside a time-bubble that was anchored to themselves using bendalloy savantism. Since the use of several time-bubbles multiply their effects, if an allomancer was to be within two time bubbles, they could run 16896 ft in 15 seconds of out of the bubble time. (OTBT) The acquiring of multiple time bubbles could be done through the use of Ettmetal, (more likely) or by attempting to cram multiple bendalloy savants into the same time bubble. (less likely, albeit more fun) The multiplication of time bubbles for an increase in distance over the same amount of time could feasibly increase until an allomancers was inside of 10 individual overlapping time bubbles. At this point, the allomancer would be running at a speed of 1889785610 or 1.88*10^10 feet per second in OTBT, which is marginally greater than 9.836*10^8 ft per second, which is roughly equivalent to the speed of light. Now that the allomancer has attained the speed of light relative to the people outside the bubble, they simply need to leave the bubble and they would be stripped of kinetic energy, returning them to the speed of 2112 ft per 2 minutes. (Assuming the noise the bubble makes when you leave it that fast doesn't kill you outright)
Edit: 300th post apparently