Tuning a different turbo and IC from a scratch is very difficult. I don't know what you are using as a starting point, but you might want to try the VF34 map on the
http://www.osecuroms.org site, as the VF39 is pretty close. IIRC, the VF39 spools a little sooner and has slightly less top end than the VF34.
You can use your MAF signal as a very general gauge on whether you are making more power or not. I wouldn't use it as a target as you are not going to hit your current peak maf throughout the rev range. Just remember, that your airflow will be affected greatly by outside temp and heat soak, so try to do run in the same outside and driving conditions and on the same road.
As far as timing, it can be tough to tune on the road. Generally speaking, your timing will start off high in the lower rpm and gradually drop until you hit peak torque, where it will be its lowest and then start going up again as you increase rpm past that. When you improve the efficiency of your motor, you actually need less timing. There is a point, in increasing timing, where you are actually making no more power and are just getting closer to the knock threshold. Yet the best power is usually made fairly close to your knock threshold, so it can be tough as well as risky balancing act. Of course, on a load-bearing dyno, you could see exactly what increasing timing will do to your power at any load/rpm site.
I would add timing to your ignition correction map first and do it slowly with lots of logging. Personally, I'm waiting until they crack the RAM giving us the ability to view the fine learning corrections, will tell you exactly where the ECU is removing timing due to knock (at a particular load/rpm). Until then, I'm not making major changes to timing as it is the quickest way for your motor to go BOOM, especially if you rush the process. Even same year WRXs with the identical mods, oil, gas mileage everything can end up working best with different levels of timing. That is why you can see big gains with a custom tune on a dyno. So there is no set of rules that says you can add x amount of timing here or there. It is a matter of finding load/rpm sites where you never experience knock, under any conditions, and slowly advancing the timing and monitor knock and seeing if you are making more power there.
