reverse sort is optimized to do the reversing in sort itself, so any difference in speed should be very minor.

Update: Corroboration:

>perl -MO=Concise -e"@b = sort @a"
...
7           <@> sort lK ->8
...

>perl -MO=Concise -e"@b = reverse sort @a"
...
7              <@> sort lK/REV ->8
...
[download]

Update: It turns out that sort simply reverses the array at the end. But because it does so in-place without creating any stack frame, it's faster than reverse.

if (PL_op->op_private & OPpSORT_REVERSE) {
    SV **p = sorting_av ? AvARRAY(av) : ORIGMARK+1;
    SV **q = p+max-1;
    while (p < q) {
        SV *tmp = *p;
        *p++ = *q;
        *q-- = tmp;
    }
}
[download]