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The OPA551 pins 1,4 and 5 connect internal to the metal package leadframe. Each of these pin's connection point are at different corners. The reason pin 4 is specified as the primary current carrier on Pg. 17 has to do with directing current flow out through this pin which appears to produce optimum performance. If pins 1 and 5 are connected to a separate, floating copper thermal mass, then the primary current can't flow through them and it is forced to flow through the pin 4 connection.
Although I can't find anything specific about the mentioned performance reduction, I suspect it is with regard to the op amp's distortion performance. I wouldn't expect much of a change in it if pins 1,4 and 5 are connected together as Fig. 34 shows.
