It would be really good if someone who can already compile custom kernels for any MT6589 phone, could make that little modification and check the results - worst case scenario, the chip would get too hot and break forever... but since it seems to have a lot of temperature monitoring in place, I don't think that would happen.
I could do it myself, but I'm not going to download 8 GB of Android source, on my limited home connection, just to apply the MT6589 patches available on GitHub, which probably wouldn't work out-of-the-box since they are for a Acer phone and mine's a Zopo 910.
Bothering people at Zopo to do it is out of question, since they are interested in selling the Zopo C2 which comes with this "turbo" kernel config.
As for the GPU clock, on this kernel source there doesn't seem to be anything controlling that based on the MTK_FORCE_CPU_89T define. But this may be just because this is an old source tree - the one they have been distributing for their clients more recently, for the 6589T, may already have the changes for the GPU clock.
The ideal situation would be to have a kernel which can allow all the possible CPU and GPU frequencies, and these would be controlled and limited on the userspace.