It took me a few minutes to find it, but what you want is the calculateOverclockedness method in GT_MetaTileEntity_BasicMachine:
Multiblock machines handle overclocking individually (e.g. the EBF also considers the coil type when determining speed and energy consumption, and the Cleanroom is not subject to overclocking), the code for them is in this folder:
Ok, I googled for the actual details of making changes; opened gt5u 5.09.31 like an archive, found needed file and through Recaf changed 2 on 4, then overwrote the original file, but that didn't work(compressor and extruder, as examples, became only twice faster). I think, it requires proper recompiling, what I unable to do. So, if you have free time, could you properly rewrite all MaxProgresstime(including Multiblock machines) from 2 to 4? I'll be very pleased.