Caffeine has been used in performance enhance for a very long time ... for good reason it works ... it will not directly increase testosterone but if you can lift more or more intensely for longer peroid of time the effect is the same ...
In general i would have to assume that anything that boosted T levels above where the body thinks they should be would cause a down regulation of T production. As seeking balance is one of the things the body does whether we like it or not.