Stillen SC - put on your seat belts

My car cost me 105,000 dollars. Just to get the standard Stillen/GTM kit here would be around 13,000 dollars. My friend who posted here 530 or so WHP paid these amounts for his kit (GTM). The short is that someone bought the Stillen kit and decided not to install it. He is willing to sell it to me for about 5500 dollars. That's cheap for here; very cheap. I know that the kit won't make the same HP as the GTM; although, I am not really sure why (not technical at all.) I just know how to drive. What would you guys do? Either no kit (can't afford GTM here), or get the Stillen?
I think I was misunderstood in my original post. My intentions now are to install the kit as is and with a proper tune get the max HP out of it. You must realize that because of the prices here car upgrades are a very grey area. Almost everyone in the states knows the basics such as how to change the oil or small repairs. Here it's the exact opposite. As to the guy that said that I didn't do research; well I did and my pockets aren't deep enough for the GTM (13000 dollars.) What I would like to know from the forum members is what realistic HP I'll be gaining with a custom tune and whether the Stillen kit is a good and reliable kit for what it is. If anyone says anything different then maybe I'll say "screw the kit" and I'll continue stock. The most important thing for me is reliability; meaning a daily driver with almost no hiccups.
Thanks