Open source tune is good but Vipec is a step above open source.
A full 32bit Vi-pec ecu is always better than the stock 16bit GD unit.

We offer open source tunes at Evolution R. However, Vipec comes with awesome datalogging for drag and circuit use which open source doesnt.

A good example will be Simon's Vipec GD vs Meatbag's Verso-tuned Sti. Same power, but Roughstilin's time on the drag strip is less by more than half a second as the Vi-pec delivers smooth torque and power through the ability to datalog extensively.