A coauthor of mine used the expression "what's more" in a scientific paper. My gut feeling was that this is not a commonly used expression in formal writing, but I could not find clear evidence for this so far.
The formal academic equivalent is moreover:
in addition to what has already been said; furthermore
Or, indeed, furthermore
in addition; moreover
In general, I would avoid contractions in formal writing and "what is more" might come across as conversational. I haven’t seen "What is more" in many academic papers, but "moreover" and "furthermore" are pretty well used. Google ngrams plots show this applies to other literature, too.