There is something in the firmware or hardware the prevents clock rates over a certain frequency. Voltage modded devices are running at a --zeus-clock of 381 (the suspected maximum)
The speed limiter cannot be overcome in the mining software.
Cooling Mods
Cooling improvements are recommended/required for volt modded devices.
See the following posts for examples of fan mods...
I could have sworn there were recommendations for thermal paste and heat sinks for the ASICs, but I can't find them. Might have gone away with the purge.
I think everyone can agree that adding heatsinks to the outside of the case/heatsink will add surface area, but ultimately does very little for keeping the chips cool.
Fan Oil Change
See the OP for how to fill or check the oil on your fans.