Yes, American Hophornbeam is also known as ironwood. But be careful, there is also an American Hornbeam that is NOT related. It too is known in some areas as ironwood. It's also known as blue beech or musclewood. It's best to use the name Hophornbeam to help eliminate any confusion.
I concur on the hophornbeam comments. It's awesome stuff. It's like albino osage. That reminds me, I need to go find some more
It's true that the bark on whitewoods peels more easily any time the sap is up. The reason some folks wait until late summer to cut their chosen tree is so that the present year's growth ring(which, ideally, will be the back of the bow) has grown to near full thickness, which may make it more durable.
As to whether a pronounced crown is a bad thing... It depends... it does raise some issues. For one thing, it makes it harder to build a wide bow or 'flatbow'. The smaller the tree and the wider the bow, the harder it gets. The inherent qualities of various woods require that some be designed wider, others can be made more narrow. Ash, for instance needs more width than hophornbeam. So, hophornbeam would be a better canidate for a smaller tree/higher-crowned stave.
But before you go running out there and cut a bunch of 3" trees, keep in mind, there are other benefits to the bowyer in flatter bow backs via larger diameter trees. Staves from really small trees can leave VERY little wiggle room in layout. And construction-wise; twist, lateral and longitudinal misalignment, any undulations in the surface of the back, or inclusion of any knots or wavy grain, etc... all of the effects/challenges of such imperfections are harder to deal with in a more highly-crowned stave than in a flatter one.
Don't get me wrong, good bows CAN be made from small diameter trees, even root suckers and overgrown watersprouts, but they're generally more challenging and one thing the novice bowyer doesn't need is another handicap.
For the general flatbow design and most variations, trees of about 7-8" and larger at the large end, including the bark, are sufficient for a relatively flat back.