Learning the stroke order probably does not help with learning the spoken language. However, with regard to the written language, the arguments that come closest to cognitive benefits are related to memorisation (including motor memory) and the ability to recognise characters. See the papers I found below.
A paper by Law et al from 1998 points out that the two traditional arguments for learning the stroke order were:
- better calligraphy
- it serves as a memory aid for the exact reproduction of the character
Law et al came up with a few other arguments for learning the stroke order:
- "stroke reversals occur in directions that prove to be difficult to draw and thus would lead to poorer and slower handwriting for right-handed scribes" (this is not directly relevant to learning),
- "the number of strokes in a character is commonly used as an indexing key in Chinese dictionaries" (this is relevant to learning in sofar as you need to look up characters in a dictionary without knowing their pronunciation; however, software that supports handwriting recognition can deal to some extent with poor character writing).
The children in the study by Law et al said that they found stroke sequence important, but this perception conflicted with the stroke order errors they made.
A paper from 2010 by Tian et al claims that a "correct order may bring benefits in both memorizing the shape and crafting a better-looking handwriting" but the authors do not back this up with research.
A study by Chen et al (2013) found that when non-native speakers of Chinese get instruction about radicals (both phonetic and semantic radicals), they perform better than a control group with regard to "radical recognition, semantic radical awareness, and phonetic radical awareness". They also had a better knowledge of stroke order than the control group. (Note: the instruction on radicals included instruction on the stroke order of the radicals.) However, this does not show that learning the stroke order as such is beneficial to learning.
A document by Discover China says that learning to write Chinese characters will help you understand their components (i.e. faster than just reading them or inputting them through pinyin or zhuyin fuhao). However, this does not require that you learn the exact stroke order.
A paper by Guan et (2011) points out that "the advantage of handwriting may have a sensory-motor
source", i.e. handwriting creates a mental model of the characters combined with a motor memory and motor memory can last for a long time. However, it is not clear to me why this motor memory would require the correct stroke order; it can probably be based on any stroke order that you use consistently.
There is also a summary of another study that says that, "All studies confirmed that when the segments of a character (...) were presented out of normal sequence, the subjects took longer to identify the character, or had difficulty identifying the character, or couldn’t identify the character."
PS: To avoid confusion: I am not questioning the usefulness of learning the correct stroke order. I have experienced the effects of sensory motor memory myself. But the studies don't prove that a non-standard stroke order, when used 100% consistently, would not have the same effects. Of course, with a non-standard stroke order, you may experience some disadvantages, e.g. when using a program that supports handwriting recognition, and your handwriting may look worse.