If you wanna mess with the factory ecu settings you have two options. First of you can opt for an AFC (air-fuel control) this allows changes in the fuel curve and changes how the mass air flow sensor adjusts the fuel curve per the amount of air going in. It's a pretty easy install, I think somebody's actually got the diagram for the wires you have to piggy back the unit into.
The second option is a piggy back ecu which plugs in in-between the stock ecu and the main wire loom. Nice add on and allows complete adjustablity for everything from timing advace/retard points to fuel curves to mass air readings and everything inbetween. If you're new to tuning and have never used one have it prof. installed, because you run a high risk of detonating the motor by just "playing" with the settings. Actually there's a third option but it's usually race apps only which is a stand alone engine mangament system. This generally cost's mega bucks and is super flexable. If you're just looking for some more flexiblity than the stock ecu allows go for an AFC Apex-i probably makes one of the best systems around and it's inexpensive at around $500.00.