The phrase 'as implausible' is correct and usable in written English.
You can use it to describe something that is unlikely to be true, for example: "The story seemed as implausible as it was entertaining."
Actually, the plot isn't as implausible as it sounds.
This claim isn't as implausible as it may seem.
This is not as implausible as it seems.
Although some philosophers and linguists have embraced this view, most regard it as implausible.
(It's not as implausible as it sounds — ask Dostoyevsky, or Jung).
Allan Houston raising his voice seems as implausible as hockey fans at Carnegie Hall.
The revival of a defunct bookstore may not be as implausible as, say, the opening of a buggy-whip factory.
The result for a sceptic is that they should have no issue with discounting God as implausible.
This wasn't as implausible as it might sound: in those years, bit players were often plucked from the crowds that gathered at the studio gates.
Such an outcome is not as implausible as it sounds.
And it's not nearly as implausible as you might think.
