It makes so many champions that have been shut out of serious play for the last two years actually viable.
However, some of its effectivness feels like just raw power, and perhaps some of that is deserved. But usually when I see this kind of strict power buff to a summoner, it gets watered down or removed soon after.
If it turns out that you can't keep it because its too powerful, I suggest you backload the movement speed so that it starts at 15ish% and goes up to 50ish% at the end of the ability. That provides an interesting decision between going in as soon as you use it to get the most out of the duration, or waiting for a bit with it ticking away in order to build up speed so that you can't be peeled when you do commit. Deciding which to do would involve evaluating how long you think the team fight will last given all the factors like team comp and positioning, and offers a chance for skill expression.
The whole issue with having fighters that can't be peeled when they commit is that there isn't generally any recourse-- but if ghost power was backloaded then you could at least see it coming (since it wouldn't ramp up until 3 or 4 seconds have passed) and prepare your cooldowns and positioning beforehand to best handle it. It also provides a way for the fighter's allies to work alongside him to make the most of it, by disrupting and dividing them before the fighter does go in so they can't coordinate their efforts to counteract the ghost.
Flash feels too abrupt to prepare around. If amumu flash ults there is going to be some time between seeing it happen and deciding that you need to follow up because you had no indicator that he was about to ult unless he spam pinged (and even then, pings can be a bit ambiguous with strangers). Ghost feels much less abrupt and quite frankly an ally with ghost is easier to cooperate with, assuming of course that flash isn't so much stronger than ghost that the team ergonomics of ghost are eclipsed.
And the fact that flash doesn't telegraph means that certain flash + ability combinations are definitively toxic to overall game health. Flash is kept because it enables "flashy" plays, but is still gated by a large cooldown because of its warping influence on balance. Ghost (especially a backloaded ghost) also enables cool plays and doesn't have the same distorting effect on balance and game health. And beyond that, ghost actually enables cool interactions between teams BEFORE they collide in a team fight, whereas flash as a mechanism completely removes those interactions if you pair it with a large aoe cc ult.
Which is why I think you should keep the buffs, in one form or another.