From Bruce Roe 

The basic purpose of thermal contact, is to minimize the
cross over distortion in the Ip booster.  To make them the
SAME temperature, rather than heat sink them.  Failure 
to do this probably will have little effect; the original 
design worked with atrocious distortion.  You could have 
some excess current flowing through Q3 and Q4, but 
probably not enough to do any damage.