Alan, if you look at the definition of contrast index (see page 9), which is/was also used for ISO speed determination, you see that a density of 0.1 over b+f level is used as threshold. If you look at the typical curve, there is discernible detail below b+f+0.1, and forced development will make this extra detail visible in enlarged prints.

So yes, you will gain a bit more shadow detail with push development. We should still not call this "speed increasing", because speed is defined for a specific C.I.