Even after so many years, Gmail still doesn't support push notifications in the Mail app, but FastMail threaded the needle to do it. Such a wonderful change; works great in my short testing. (I had to toggle the account on/off a few times to force it, but their blog post promises it propagates out automatically, too.)
Its actually enabled for all accounts and fully supported. We just haven't advertised it widely yet because its a big, complex feature and we've got enough people travelling at the moment that we didn't want everyone to start using it all at the same time!
Documentation is at: https://www.fastmail.com/help/clients/applist.html
You can see that it is leasted on our pricing pages: https://www.fastmail.com/signup/
We'll announce it officially in the next 2-3 weeks.
https://support.google.com/a/answer/135937?hl=en
So, when comparing paid and paid accounts, there is not a difference here.
(Of course, the GMail app gives push for all GMail accounts.)
It has been available in beta for qiuite some time really.[1]
You can start using it now if you like. No need to wait.
I'm a fastmail subscriber and this is my only way of syncing contacts on my Android-devices right now.
[1] http://blog.fastmail.com/2014/12/22/carddav-beta-release/
Furthermore, GTalk is getting phased out in favour of proprietary Google Hangouts. GTalk was always quirky as well, and fails to support many XEPs (XMPP extensions).