I'll be honest, and say that while SymStretch is getting there, there are still some issues in some cases (mainly around email and filtering) which can cause some problems, so it's not something I'd particularly call production ready.
As far as timescales go, I'd like to cautiously say that we'd have it ready by the end of June, but can't really make that promise. I'll ask Andrew here who's coordinating the release of it though how confident he is that it will be available before then.
However, it's worth noting that the the main issue we're encountering with customers moving to Debian Stretch is that a significant proportion of sites and older CMS's aren't very happy with the changes in PHP 7.0 (there are quite a few) so while it does provide a speed increase in some cases, often need to apply third party patches and so on to ensure that sites stay working, and potentially even fix issues in plugins and so on - Magento being a good example.
To be honest, the migration process (assuming you control the domains) is fairly simple, and typically a case of copying the database over along with the relevant directory from /srv, and this is the same with both Wheezy to Jessie and Wheezy to Stretch, so even if you would need to do it multiple times, it's not a huge amount of work involved.
As far as the in-place upgradego, Wheezy to Jessie is usually fairly smooth, as is Jessie to Stretch, although it's typically a 50:50 probability of there being a problem each time theres a dist-upgrade between incremental versions, so we suggest against it where possible, and I (personally at least) wouldn't try going straight from Wheezy to Stretch.
In the end, I'd probably suggest migrating to Jessie sooner rather than later (via a new server if at all possible), and then repeating that later on if PHP 7.0 is still required and everything is compatible with it.
As mentioned, I'll pass this to Andrew here to comment on an estimated timeframe for SymStretch.