Mining for nil-transaction blocks only - gaming the incentive scheme by rogue miners / consortium?
(I'm certain this has been asked before, but I can't find the previous question.)

That would be true, except for the presence of the nonce and extraNonce fields. The point of these fields is that you can rehash the block without actually hashing the whole thing. On 2^32-1 out of 2^32 times, that just means hashing the last 16 bytes of block header, which can be done in 3 compressions. (That's one for the block header, and two for the following SHA256 hash. Remember, Bitcoin uses SHA256d.)

A single mining attempt takes (3 + ((log(2, number_of_transactions) + 1) * 2 / 2**32)) SHA256 compressions, on average. Having a thousand transactions in the block takes an extra 0.000000004 compressions per hash over having no transactions in the block.

The effect of all of that is that you mine at essentially the same speed no matter how many transactions you include.