[plt-scheme] Has there been an improvement in the GC and/or virtual memory manager since v369.100?

Today I was running a benchmark on several functions, which I had run before in
v369.100, and I noticed a distinct difference in be the behavior of these
functions as they were given ever larger lists at the end of the run, when they
are given lists that previously would have caused at least one of them to
exhaust the available memory.  On the final run they are given lists which
contain on the order of 156,000,000 symbol?'s. I'm not sure how much memory is
required to store a quoted unbound symbol in a list.  But let's just you use a
two 32bit words for each pair, as a minimum, for the pointers to the car and
cdr.  That would be eight bytes, making the memory requirement for a single list
on the order of 1gb, and the function is required to construct a flattened copy
of the argument list, which brings the total, conservatively, to something like
2gb's of memory required.  I'm running this benchmark on a Mac Pro with 4gb of
physical memory, of which only a portion of the last gb is available for use by
anything other than devices.  So, I'm not surprised that the program would cause
a heap allocation error.  

Whats surprising is that, instead of crashing as in v369.100, the run times
explode beginning at list lengths (not to be confused with the number of
elements in the list) of 1.8 million, and the hard drive is heard to be
thrashing, where as I normally can't hear the drive at all.  The benchmark
completed without a single error, but there was a distinctly non-linear growth
in run times between lists of length 1.8 to 2.0 million.  If there was a change,
it is a welcome one.  Just curious if I was seeing the benefit of an
improvement, of if something else was up?

Thanks,

--kyle