Optimization: to predefine a piece of heap memory before using several objects - GAINS?

Are there big gains in PERFORMANCE by distributing heap memory in advance and gradually increasing it?

Consider a VERY simplified example below:

byte * heapSpace = malloc (1 000 000);
int currentWriteSpot = 0;

struct A {
  int x;
  byte * extraSpace;
  int extraSpaceLength;
};

//a1 needs 10 bytes of extra storage space:
A a1;  
a1.x = 2;
a1.extraSpace = heapSpace + currentWriteSpot;
a1.extraSpaceLength = 10;

currentWriteSpot += 10;

//a2 needs 120 bytes of extra storage space:
A a2;
a2.x = 24;
a2.extraSpace = heapSpace + currentWriteSpot;
a2.extraSpaceLength = 120;

currentWriteSpot += 120;

// ... many more elements added

for ( ... ) {
   //loop contiguously over the allocated elements, manipulating contents stored at "extraSpace"
}

free (heapSpace);

VS:

...
a1.extraSpace = malloc ( 10 );
a2.extraSpace = malloc ( 120 );
a3...
a4...
...

//do stuff

free (a1.extraSpace);
free (a2.extraSpace);
free ...
free ...
free ...

Or can it just add complexity without significant performance gains?

Thanks guys!