Looks like
firmware 5.3.8 for KPW is out. I read (see linked thread) it allows multiple dictionary lookups, which is a really good new feature (well, quite old indeed, for people using jbpatch).
Being possible to hyphenate AZW3s with a
calibre plugin, the only very missing feature is a decent margin customization, like that provided by jbpatch.
In fact, at the moment, a jbpatched device is still better than the original firmware alone. I have multiple dictionary lookup, I have hyphenation both on AZW3 (thanks to the calibre plugin) and on Mobi (thanks to jbpatch) and I'm able to fully use the kindle screen (thank's to jbpatch, again) without those stupid large white margins. I won't upgrade my Kindle to newer firmware, yet.