Batting stats for all current test captains.
I've given their average overall, their average when not captaining and their average when (since) captaining.
Michael Clarke
Overall 51.82 (8240 runs - 27 centuries from 105 matches)
Captain 60.05 (3543 runs - 13 centuries from 37 matches)
Non-Captain 46.97 (4697 runs - 14 centuries from 68 matches)
Mushfiqur Rahim
Overall 32.43 (2173 runs - 2 centuries from 38 matches)
Captain 42.52 (978 runs - 1 century from 14 matches)
Non-Captain 27.15 (1195 runs - 1 century from 24 matches)
Alistair Cook
Overall 46.51 (8047 runs - 25 centuries from 102 matches)
Captain 47.02 (1834 runs - 7 centuries from 21 matches)
Non-Captain 46.36 (6213 runs - 18 centuries from 81 matches)
MS Dhoni
Overall 38.77 (4459 runs - 6 centuries from 83 matches)
Captain 42.18 (3037 runs - 5 centuries from 53 matches)
Non-Captain 33.06 (1422 runs - 1 century from 30 matches)
Brendon McCullum
Overall 38.09 (5219 runs - 9 centuries from 84 matches)
Captain 50.95 (1121 runs - 3 centuries from 14 matches)
Non-Captain 35.63 (4098 - 6 centuries from 70 matches)
Misbah-ul-Haq
Overall 48.75 (3218 runs - 5 centuries from 46 matches)
Captain 61.38 (2210 runs - 3 centuries from 27 matches)
Non-Captain 33.60 (1008 runs - 2 centuries from 19 matches)
Graeme Smith
Overall 48.72 (9257 runs - 27 centuries from 117 matches)
Captain 48.32 (8651 runs - 25 centuries from 109 matches)
Non-Captain 55.09 (606 runs - 2 centuries from 8 matches)
Darren Sammy
Overall 21.68 (1323 runs - 1 century from 38 matches)
Captain 22.43 (1032 runs - 1 century from 30 matches)
Non-Captain 19.40 (291 runs - 0 century 8 matches)
Brendan Taylor
Overall 35.00 (1260 runs - 4 centuries from 19 matches)
Captain 52.37 (838 runs - 4 centuries from 9 matches)
Non-Captain 21.10 (422 runs - 0 centuries from 10 matches)
Some interesting reading.
Of note, IMO:
Brendan Taylor of Zimbabwe. Averaging 21.1 in 10 matches, has belted 4 centuries at 52 since taking over as skipper. Mushfiqur Rahim is another of note.
Smith's amazing record - 109 matches as captain after serving an 7 game apprenticeship. Leading the way he does with such minimal technique. (I actually started this two nights ago before I heard about his retirement - a great career, a shame that it ended on such a poor note form-wise, though that's often the way sport goes)
And the first bloke's average as captain hasn't been too bad, either.
Clarke and Misbah have really blossomed since captaincy. Amazingly, Misbah has passed 50 24 times in 47 innings since taking over, though Clarke's centuries have been impressive. He has a touch of feast or famine about him.