"Astonishing enough" is correct and usable in written English.
You can use it when something is unexpected or remarkable but not to the extreme (i.e. not "astonishingly"). For example: "She had worked hard enough to qualify for the job, but she never expected to get the position astonishing enough."
How Greenport got to this point is astonishing enough.
THE transformation of China's capital over the past few years has been astonishing enough.
That was astonishing enough, but then Groupon snubbed the search giant, a declaration that it was really worth much more.
The scoring was astonishing enough, but this is the 32-year-old's first event since the Ryder Cup in September.
Nathaniel Philbrick's book, on which the film is based, was all about deconstructing the myth of Moby Dick and reconstructing the story - astonishing enough in its own right - of what really happened to the Essex and its sailors.
For many it was astonishing enough that the great horse became the first to regain the Blue Riband of the jumps back in 2009 – a year after being apparently broken by the ferocious strength and will of Denman.
Poet and frontman Gord Downie delivered an astonishing performance impressive enough on a regular day, devastatingly powerful for a man with terminal cancer.
It is an astonishing place, with enough of the market, the forum, the circus, the gymnasium and baths in place to gain an idea of the sheer size and scale of the buildings, of how life would have been lived more than 2,000 years ago.
The crew had been expressly ordered to pick out their target visually, rather than by radar, since the explosive reach of the bomb, although astonishing, was still limited enough that to be off by a mile or two might result in the majority of its power being wasted.
As a result of this transformation, we can do something astonishing: We can take enough carbon out of the atmosphere and safely store it in the grassland soils for thousands of years, and if we just do that on about half the world's grasslands that I've shown you, we can take us back to pre-industrial levels while feeding people.
And yet I remember a student raising his hand after one of Stanley Elkin's astonishing performances (we knew enough not to sit in the first three rows because of the flying spittle as Stanley worked himself up into an actor's rage) and asking this: "Mr. Elkin, you've written a terrific collection of stories — why don't you write more of them?" Stanley's answer: "No money in it.
