I don't have such a client -- just to clear that up.
What I'm getting at is this: If BTC were highly valued, and a very clever-but-selfish developer had an exceedingly fast client (say, someone gets CUDU running on all cylinders, or hand-crafts SSSE3 for i7), would they be better off retaining the program for their exclusive use (during inflation), or offering it for 'sale' by effectively charging a fee for every find.
I suspected it would be more profitable to distribute it with a small fee, but the reception sounds a bit... less favorable.
The difficulty would just goes up and make it harder for you to generate bitcoins.
But it would go up less if you were the one with the fast client and everyone else had "normal" clients. If you distributed such a fast client to everyone, then yes, the benefit would cancel out.