Performance extremely numbing, I'm heading for the exit! (2,4)
I believe the answer is:
so long
'i'm heading for the exit' is the definition.
'performance extremely numbing' is the wordplay.
'performance' becomes 'solo'.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'numbing' with its centre taken out is 'ng'.
'solo'+'ng'='SO LONG!'
(Other definitions for so long that I've seen before include "This extended" , "Au revoir" , "Goodbye (colloq.)" , "That length - goodbye!" , "Cheerio, bye-bye" .)