sure about 4.5" exhaust? i know mine is 4.5 at the tip, but thats the tip. rest is 3 which is usual for after market.
i agree about not getting a 4" inlet and out let, would be no point and cost you more. the rest of your piping is 2.5 so even if the converter from 4-2.5 was free, there would still be no gain from having it. would need all the piping to be 4" for there to be any chance of improvement, minimal at best.
the ones from just jap are good, im running one at the moment. was a bit of a tight fit with my front bar, but its do-able and they do help on hot days, and making things a little smother.
as for turbo lag, if there was any more than usual, cant notice it at all. spools fine no extra lag.
just jap, way to go, but just keep in mind that if you are putting it in your self, the kit doesnt include the red pipe/tube and T piece pictured. so you'll need to buy a T piece and rubber tubing as well. goes from the boost controler to the inlet pipe btw.