Gotta agree with sssstew, cheap resistor box, don't touch it. They cause faults to occur.
There are only 2 real options,
i) a proper tuning box, Tunit, Dragon, PSI are a few well respected names in this field, but there are others. These are all adjustable to suit your car.
The power increases are there, but delivery will not be as smooth as a remap.
ii) a remap, this is the best way, and if you
shop around, will not cost you much more than a high quality tuning box.
power delivery will be smoother
hope that helps