If you don't have a turbo, don't bother... Unless you are ready to spend money on engine parts and blueprinting...
The gains of reprogramed ECU in a non turbo car are not a good value for money and nothing great.. eventually 20hp??
To get something out of your V6 you need to work on the air intake, fuel injectors, exhaust system, eventually get more displacement by milling cylinder, new lightwheigt pistons an conrods to increase rev range, etc, etc then change ECU to take advantage of those changes....
Bear in mind that you need a very good tuner to do this in the right way... otherwise you risk blowing the engine out. Something around £2000-£3000 should do something visible to your engine...
I prefer non turbo engines but they are not easily upgraded /modded unless your serious about it...
All of this is OK until you need to pass the MOT...
Pedro