The only overclocked 040's I have are CPU upgrades, and they are a little faster (much faster if they have cache onboard). If you want to speed up a Quadra 040 machine try to find a Daystar cache card that fits in the PDS slot.
The IIsi is particularly amenable to overclocking due to Apple intentionally crippling it. 25MHz is easily achievable (the parts are already rated for it), and some people have pushed it up as far as 28MHz. The speed difference is noticible.