1. Zero HP. Your ECU's fuel/air maps are not adjustable without modification. Adjusting these requires a device to modify the input/output signals such as an Apex'i S-AFC or a full standalone EMS like my Haltech. Be prepared to drop $1000+ for a standalone.
2. Dyno & laptop
3. Full rebuild with lower compression pistons. The stock ZZT231 pistons are insanely high compression and would instantly blow the engine if you tried boosting it. In doing so, you negate any benefit from the VVTI system.
4. Not necessary
All in all, the Celica just doesn't have any torque. In bone-stock form, my 1987 Mazda RX-7 (with only 146hp then) consistently would jump off the line quicker than my friend's 02 Celica GT-S because it has more torque at a lower rpm.
When guys talk about Powerband, it is the range between maximum torque and maximum horsepower. The Celica's powerband is between 6800 and 7600rpm, so it has to work very hard to get moving and gear changes have to be done VERY quickly and carefully to stay within the powerband. My RX-7's is 3500-6500rpm, so its powerband is nice and broad. Shifting up isn't as critical because its torque will pick it right back up. This is where your transmission's gear ratios come into play. In general, torque is a function of the engine's displacement.
Now that my RX-7 is sporting a 20B-REW under the hood, its powerband is from about 1500rpm to redline, due to 50% more displacement and the rather small twin turbos spooling up easily and keeping up till the end of the tachometer. Haven't dyno'ed it yet, so this is all based on the original specs & dyno graphs from the Eunos Cosmo.
The lesson here is that when you start extracting horsepower beyond the balance point (Horsepower & Torque in ft.lbs being about the same), the powerband moves up toward the end of the tachometer and the car simply has no torque.
If you do get the Celica, try getting it dyno'ed while completely stock. There's a lot you can learn from reading the dyno graph. Most tuning shops will show you how to read it if you ask nicely.
owner of a HEAVILY modified Mazda RX-7