There are two clocking issues here; one is running everything on the motherboard at a different speed (33 or 40 MHz), which is why some people have problems with the speed of the video memory, and which is also why people don't just swap the resistors to give them 40 MHz. The memory on all the Q605s I've seen have always been 70ns, whereas 60ns SIMMs are pretty common. My guess is that if the motherboard memory were faster, the 40 MHz resistor change would work.
The second clocking issue is bypassing the motherboard speed settings and sending a separate clock signal to the MC88920 (what I mentioned before). The page which discussed this also recommended doing the resistor swap to 33 MHz, probably so that the memory will run a bit faster. In this case, an unhappy MC88920 might give a different clock rate (somewhere between 36 and 40 MHz), but I can't imagine even the most unhappy MC88920 giving 25 MHz.
So the question is whether anyone who has swapped their 88920 for an 88916 has done a proper resistor swap, or inserted a 20 MHz oscillator signal, or both, and if they have, that'd be VERY strange... Where did you hear about these cases?