The 1.8 shares the lpfp with the gti. So far they have gone up to 450+whp on the stock pumps and injector upgrades. The only reason there is a difference on the hpfp is because of the different head with the variable lift on the exhaust. The head specs as far as flow are the same and so are the intake cams. The 1.8 uses same bore pistons with a shorter stroke crank. Pistons and rods are forged and the crank is not. That being said, torque breaks parts, not hp. Lots of cars have gone more power than that on more torque and more hp. There is no need to change the head to the gti head. The vvti on the exhaust only helps the low end. That is why you will see big turbo 1.8's making more boost from 4,500 on up. The vvti actually hurts the top end because it allows the turbo to spool up and blow it's load sooner. All you will really need to get at that power level is larger injectors, secondary injectors like a ROW car, a larger lpfp, a turbo bigger than an IS38, a larger intercooler and some meth injection. As far as what the transmission will hold, again, torque kills, not hp. While Blade has transmission issues, I didn't think he broke or stripped gears on it. I thought he had some leaks. Lots of drag launches on a high torque tune as well. There are other 5 speeds and auto trans guys on IS20 with no issues. If I'm not mistaken, you are a DSG though. So no issues. Just get a TCU tune.