This makes no sense. Acceleration occurs independent of velocity. Real life plasma thrusters will not 'lose acceleration' as velocity increases. Thus, the point you describe where the increase in velocity will become unnoticeably small will never happen, because the increase in velocity will remain constant.
Check the post, please. I never said they 'lose acceleration,' nor did I claim that acceleration 'becomes smaller.' I guess what I said was too ambiguous. I meant that eventually you will attain a velocity that makes additional increases in velocity insignificant, so let me rephrase:
This would mean that eventually there comes a point at which you are still accelerating, but the increase in velocity is so small compared to your present velocity as to be almost unnoticeable.
Here's an extreme example: say you accelerate at 7 m/s^2 for a very long time until you're traveling approximately 1/3 c. 1/3 c >> 7 m/s. At this point, an additional 7 m/s added to your velocity is almost unnoticeable, since it will be hard to tell 100,000,000 m/s from 100,000,007 m/s. Though given the potential thrust of a plasma thruster, the FS values become incredibly unrealistic and suggest this may not be the case.
You're missing the point again.
Right now, you are traveling at 200,000 kilometers per second, viewed from someone else's IRF.
If you are then thrown (accelerated) through a wall at a mere additional twenty kilometers per second, is that change in velocity insignificant? Hell no. Yet you claim it is.
Is it, as you claim, 'hard to tell the difference'? Again, hell no.
Therefore, this:
at this point, an additional 7 m/s added to your velocity is almost unnoticeable, since it will be hard to tell 100,000,000 m/s from 100,000,007 m/s
Does not compute.
Like Aardwolf said, you're attempting to compare velocity and acceleration, yet they're very different things. Your velocity has
no bearing on your acceleration. The big mistake you're making is thinking that it does.
EDIT: To clarify. Any given observer (ship, in this instance) views its own velocity as zero in its reference frame. All that it can sense is the acceleration it's under. Therefore, it doesn't matter whether it's cruising at 10 m/s or 10,000 m/s from an observers's standpoint. A thruster firing at 5 m/s/s will always feel like the same change in velocity to it, and it's always equally significant.