Well, the kit is useless in my opinion. It offers a spring upgrade that will only damage your rifle. Think about it. The only durability upgrades in this package are a new spring guide and a piston. What about the trigger box? I have seen people put this kit in, and it simply just destroys the trigger box, leaving the user with the options of either buying a new trigger box (cost around $100 to $200) or to ditch the gun and buy a new rifle.
If you are set on upgrading your fps, then you need to buy some durability upgrades. Parts from laylax, pdi, or OK will be well worth the money. I always tell people this: Do not upgrade the spring until you upgrade for durability. Here is a list:
1. cylinder head
2. spring guide
4. trigger unit or new sears
Buy those and then upgrade the spring. I recommend a PDI spring or a laylax one. The spring in the kit you have listed will wear down, and it will do so quickly (results in lower fps output). Its not made to as high of a standard. Think about it. A laylax piston costs around $45 and this kit you have listed comes in a package for $35. You get what you pay for, and replacing crappy plastic parts with a few pieces of pot metal along with a spring way too powerful for the rifle is just going to cause you problems.
Thats my 2 cents.
Edit: And if you find yourself not able to afford all the durability upgrades at one time you need to have patience. Slowly work your way up, purchasing piece by piece. Once you get those upgrades in the rifle, then you can upgrade the spring. But do not jump the gun. If you do, you will be stuck with a rifle that is broken, and you will have to buy numerous parts at once just to get your rifle back up and running.