IMO a good piggyback with get you a big chunk of the performance a full stand alone will. It depends on the base engine though and good piggybacks like the Haltech Interceptor and AEM F/IC are not compatible with every application because of trigger input support. Any piggyback is simply modifying the input signals from the stock sensors to make the stock ECU do what you want it to. Some, like the mentioned Venom, are pretty crude and are basically like the old trick of placing resistors in the water temp sensor. The Haltech and AEM are far more advanced and offer very good tuning resolution.
The Haltech has 16x16 mapping for fuel and spark. Fuel is altered as a percentage of stock injection pulse width so, with a 0% offset on initial startup you're running a completely stock map and you simply wire it up and drive away. On the dyno you see where the car is lean or rich and alter the offset at those particular load/rpm sites to dial in the mixture that gives you best relaible power. Since your stock ECU drives the car so well off throttle and we're not concerned with making more power there you can leave all those load/rpm sites at a 0% offset for quicker tuning and stock like driveability and idle. Timing is adjusted as actual degrees BTDC. Again you can leave it stock or tweak the timing at full load to optimize power output.
There are advantages to some full standalones. Some modern ECUs are very smart and will figure out that you're trying to fool it with a piggyback and jump into a limp mode. I'm not even sure if there are any ECUs like this in IT currently. We've worked with some pretty modern stuff in the Bosch ECUs in the Mini Cooper S and the Haltech Interceptor works great. However, Mazda's turbocharged Mazdaspeed cars are notorious for figuring out and defeating piggyback systems. In cases like this a full standalone is required. Sone standalones like the new Haltech Platinum Sport 1000 alos have added features over piggyback systems in that they can control aux functions such as electric cooling fans and have very good onboard datalogging that allows you to record engine and aux input data with the ECU for later download in the paddock. Single connector CAN outputs to digital dashes are also nice. There is also the potential weight savings of using a standalone system which can eliminate a large chunk of wiring from the stock system.