Using the new benchmark, we can see some improvement, mostly in the single threaded case:

Custom mergeSort:
RESULT : DolphinSortingBenchmark::sortInt():

0.25 msecs per iteration (total: 64, iterations: 256)

PASS : DolphinSortingBenchmark::sortString(usr_bin)
RESULT : DolphinSortingBenchmark::sortString():"usr_bin":

2.4 msecs per iteration (total: 79, iterations: 32)

PASS : DolphinSortingBenchmark::sortString(citynames)
RESULT : DolphinSortingBenchmark::sortString():"citynames":

0.14 msecs per iteration (total: 72, iterations: 512)

PASS : DolphinSortingBenchmark::sortStringParallel(usr_bin)
RESULT : DolphinSortingBenchmark::sortStringParallel():"usr_bin":

0.63 msecs per iteration (total: 81, iterations: 128)

PASS : DolphinSortingBenchmark::sortStringParallel(citynames)
RESULT : DolphinSortingBenchmark::sortStringParallel():"citynames":

0.056 msecs per iteration (total: 58, iterations: 1024)

PASS : DolphinSortingBenchmark::cleanupTestCase()
Totals: 7 passed, 0 failed, 0 skipped, 0 blacklisted, 1610ms

std::stable_sort:
RESULT : DolphinSortingBenchmark::sortInt():

0.10 msecs per iteration (total: 54, iterations: 512)

PASS : DolphinSortingBenchmark::sortString(usr_bin)
RESULT : DolphinSortingBenchmark::sortString():"usr_bin":

2.3 msecs per iteration (total: 76, iterations: 32)

PASS : DolphinSortingBenchmark::sortString(citynames)
RESULT : DolphinSortingBenchmark::sortString():"citynames":

0.11 msecs per iteration (total: 57, iterations: 512)

PASS : DolphinSortingBenchmark::sortStringParallel(usr_bin)
RESULT : DolphinSortingBenchmark::sortStringParallel():"usr_bin":

0.61 msecs per iteration (total: 79, iterations: 128)

PASS : DolphinSortingBenchmark::sortStringParallel(citynames)
RESULT : DolphinSortingBenchmark::sortStringParallel():"citynames":

0.047 msecs per iteration (total: 97, iterations: 2048)

I expected that std::inplace_merge would be faster than the custom merge function, but that was not the case in the benchmark; it was actually quite a bit slower.

Seems sane to me and nice since it removes some code.