I have just started a PhD in Mathematics. Can serious mindfulness practice (i.e. vipassana meditation at least 2 hours a day) help improve academic performance? If so, can you attach any reliable studies on this?
This question appears not answerable. For example, if your vipassana practise is highly successful, your mind could become "dispassionate" towards mathematics. Instead of earning a PHD, you might become a monk & spend your life wandering among peasants in Cambodia.
Please see this answer for adaptation of Buddhist mindfulness into medical therapies called Mindfulness Based Stress Reduction (MBSR) and Mindfulness Based Cognitive Therapy (MBCT).
Those therapies are not Buddhist (as in their goals are not the goals of Buddhism), but they are based on Buddhist practices. If stress, depression and anxiety impede academic performance, then those practices can help.
That answer provides the studies on MBSR and MBCT.