if u dun mind of slightly higher fc and roar sound at highway, u can change to mivec close gear ratio lsd gearbox, with racing clutch, it will be nice to drag, from low end till high end....
Mivec gearbox is not very suitable for gsr-T application, its too short and you will have more wheelspin on full boost that you may need to change tyre everytime you change engine oil for those with heavier right foot. I would say stick with what you have. An evo gearbox would be one of my candidate as well. Bare in mind that the usual 2wd gearbox have smaller gears therefore not as robust as the 4wd unit.