Your link does not point right but I assume you meant
viewtopic.php?f=34&t=87869#p495753 If you didn't use "transient" as section name, then you are experiencing something else - something that is a bug, omission, undocumented - and as such subject to change, so don't rely on it in your code. Same goes on being able to store >16KB in the registry because of the gzip implementation. I say don't rely on that, be prudent in your choice.
On a side note, amusing thought - formatJSON() will likely give you a close estimate of how big the registry storage (keys and values) will be pre-compression.