First things first, the stock turbochargers won't support 450hp worth of airflow. In fact, at the stock 320hp of a stock 3SI, they are just about maxed out for airflow. So, if you want to get an extra 100hp, you will need to either add nitrous, or order $3000+ worth of new turbos.
Spark plug wires don't gain HP. Bad (as in corroded, broken) can cost you HP, but you don't gain HP with them.
Boost control won't help since you'll be limited to gaining another 15hp give or take from it. It's a downer to hear, but it's true. Mitsubishi put incredibly small turbos on the engine since they are only powered by 3 cylinders each so that they would spool early and get good low end torque (since the pig weigh over 4100lbs and has a V6). Unfortunately, turbos good for low end torque make jack for high HP.
The stock engine runs either 9 or 11psi of boost, depending on the year (93 and older is 9, 94+ is 11). You can safely go up to about 13.5-14psi on the stock turbos, but it doesn't gain a whole lot. It creates more heat than power, which can cause more damage than it's worth risking.
A quick google search turns up these results for power increases:
($300) K&N Filters:3-5hp
($650+) Cat-back exhaust:8-10hp
($100 manual, $600 electronic) Boost control, up to 14.5psi: 30-50hp.
($300) High flow cats: 3hp
($300+) High flow downpipes: 8hp
So at this point, you would have spent $1650 and gained, modestly calculated, 52hp, or generously, 76hp.
From here, to gain any more, you will need to upgrade the fuel injectors ($65 each x 6) minimum 550cc/min, fuel pump (255lph for $129), and some sort of tuning solution such as an AFC ($500ish). You would also need a new clutch at this point, because the stock clutch doesn't hold well past stock power levels ($350+). These would net you exactly 0hp, because they are supporting modifications for what comes next. (Total for this part: $1370 roughly)
What's next? You can port and polish the heads, or you can upgrade the turbos.
PnP is about $1500 and requires serious downtime, and you can gain 10-30hp, depending on how good of a port job you get.
A pair of 14b turbos would make a decent dent and add probably 30-50hp, maybe more. Of course, to get a pair of good ones will be $600-800 each, plus I think the exhaust flange is different, so you'll need new downpipes, etc., plus good custom tuning (usually about $300/hr). So figure on $2000-2500 for a cheap turbo upgrade.
Total so far? : $5020. Not quite $10,000, but it'd have gained, if you're lucky, about 110hp. If you're not lucky, about 80hp.