Depends what you mean by "tunning" it.
If you have the generic CVT, without manual shift override, then yes it is possible to install GTS manual override. I forgot who in this forum has done so, but it has being done. He installed a GTS shifter assembly, and able to shift like an auto-stick. Once that's done, it is also possible to add Lancer's magnesium paddle shifters to the steering column - same ones used on GTS/RA/Evo.
What this does for you:
1) allows you to manually pick your gear, rather than listening to the CVT whining.
2) paddle shifter allows double-tapping down shift. I find with the GTS, single down shift still does not give enough torque when passing others on highway. Most of the time, need double- down shift. Very rarely, I also needed triple down shift.

Another mod you can do, is change to a bigger transmission fluid cooler. This is also a common mod several folks on this forum have done and documented. Stock lancer's trans cooler come in roughly 3 types. The highest capacity one is used on GTS/RA/Evo, and is actually undersized for the RA/Evo. It is also mounted in a part of engine bay that does not get fresh RAM air. So most upgrades is to a non-Mitsubishi, after market, bigger cooler (bigger cooling area), and sometimes with a motorized fan on it which is activated either by a manual switch, or thermostat switch.

After you done all that, your next step can consider a turbo.