How does one go about locking down transient API requests for multiple threading? Has anything done some benchmarking, just how much can the transient API handle for multiple concurrent requests?
Under normal circumstances Transients API is essentially thin wrapper on top of Options API.
The difference is that (unlike plain options) transients might make use of object cache, if plugin that provides such functionality is installed.
So answer to this – transients perform as well as your database does, unless you change them to use another type of storage. Then it’s as good as that storage.
Also how would one go about forcing the transient API using WP’s built in cron? For instance I have a transient firing on page load that I gather statistics from ( another seperate function) but if a page isn’t visited then the transient is not updated, how would you loop through all the pages/posts using transient API to force an update?
I am not sure what you mean here. Transients are merely storage mechanism. They are used to store and retrieve [expiring] data.
Anything other than that you will need to implement in your own code.