Difference between Cantonese and Mandarin: using 係/是 to link noun with adjective?
我(高)是高 in Mandarin and 我(高就)係高 in Cantonese are both common phrases. They are always followed by a contrasting statement 


Simply put, you have to follow "我是高" or "我係高" with a "but"  

Example:

"我(高)是高, 但還沒高到可以打職業籃球" (I am tall, but not tall enough to play pro basketball)

Same goes to Cantonese: "我(高就)係高, 但仲未高到可以打職業籃球" (I am tall, but not tall enough to play pro basketball)


  Both "我是高, 因此可以打職業籃球" and "我係高, 高到可以打職業籃球" are grammatically incorrect because there are no contrasting statement. 


One difference:


  "我高是高"(It is true that I am tall) is not usually reduced to "我是高" in Mandarin , even it is not technically wrong;  on the other hand, "我高就係高" is routinely shortened to "我係高" or even "我高" in Cantonese"