Speedware also makes their removable harness bar although it costs more than the autopower roll bar. I may get one next year for autox use. I'm not going to say the chances of a WRX rolling over at an autox are zero, but it's pretty close. If you are doing alot of track stuff, then you might as well just get the roll bar.
There are multiple ways that you can cinch up the stock seatbelt if that's all you are looking for. You can do the method where you tug on it before a run, or you can get it tight and then twist the end 360 degrees to keep it from retracting. I have another little thing I put the end of the belt near the tensioner to keep it from retracting.