well in my experiences, custom kits are cheaper since companies will add on money for profit often, and you can get the parts individually at your own pace and look for deals. for example, here in america we have a kit made my Stafford Fabrications which is the #1 kit out there right now. the gts is still under development i think, but it should be released soon if im not mistaken.
the stafford kit uses a t3/t4 hybrid, and if you have intermediate knowledge about Turbos, they aren't efficient for the levels of boost for stock internals. right now im planning on running a T3 Super 60 trim Turbocharger, which is pretty efficient around 3000 rpms - the range of where i do my driving at. a t3/t4 may work good, but not as efficient as it should be at 6-7psi. basically what im saying is, pre-fabricated kits straight up come either with everything or without everything, whereas custom kits you can include everything you want to the cheapest prices you can find and you can use the parts of your choice. though pre-fabricated kits have a significant amount of r&d put into them, you can get a custom kit working just as efficiently if you spent the time to effectively plan it out.
like Illusive said, learn to shift faster. 5500 rpms is real low. usually on the 1-2 shift you want to land in lift so you will get no "lag" that you are experiencing.
v-afc's are specifically for vtec engines, a technogoly our cars do not have(although vvtl-i is somewhat similar). you would need the Apex'i Power FC to manage the fuel system tweaks, etc. after you install the Turbo, no matter how low of a PSI you are running. more air into the engine means more fuel which means tuning.