Switch & Calculate Issue

You just have to work this until you get your logic correct.

For example, by your stated rules, Donald Duck gets an N/A because his FY17 score is NOT less than 3.

Performance? = 
VAR Rating = SWITCH(
TRUE(),
[FY16]>=4 && [FY17]>=4,"High Performer", 
([FY16]>=3 && [FY16] <= 3.5) && ([FY17] >= 3 && [FY17]<=3.5),"Core Performer",
[FY16] < 3 && [FY17] < 3,"Low Performer",
"N/A"
)
RETURN Rating

For your second one, maybe something like this:

Performance2 = 
VAR Score = [FY17]+[FY16]/2
VAR Rating = SWITCH(
                    TRUE(),
                    Score>=4,"High Performer",                    
                    Score>=3 && Score <= 3.5,"Core Performer",
                    Score < 3,"Low Performer",
                    "N/A"
                )
RETURN Rating

These formulas work for their defined rules, but you will have to edit in your own rules.