ICOT,
Tachyons are, to this point, purely hypothetical. I’m not saying that they don’t exist, mind you… just that they are, to this point, purely hypothetical.
I could be wrong on this, but I’m fairly certain you need to go faster than 25K MPH to see the effects of time dilation.
Lastly, of course it’s possible that FTL travel can happen one day through technological means. However, your analogy to supersonic travel* is flawed. No one denied that an object could move faster than the speed of sound, they merely theorized that a structural object (such as an aircraft) could not withstand it. I’m sure they would have agreed that with better engineering and materials, it would be possible.
However, nothing (as we presently understand it) can go FTL. Any technological advances that will allow FTL travel will probably not involve altering this law of physics, but rather with warping space and/or time in some way.
The Wolf
* Bonus question: What was the first man-made object to move faster than the speed of sound?