The "reinforced" in the kit description does not mean the upper gearbox body itself is any different than the one that came in your stock ICS M4A1. All it means is that some of the parts inside (Spring, Spring guide, Piston, Piston & Cylinder Head Set) are an upgrade to the stock components. With that said all you really need to replace in your current set-up would be those items as I listed them separately in the previous post.
Since the money is not too big of an issue maybe this would be a wise purchase. I am only saying this because having a backup assembled upper gearbox (I have 3) will keep you in the fight should something break or a different FPS is desired. They are very easy to swap out in the field or at an event without any tools. Order the kit plus the bore-up parts (4 Port Cylinder, Piston & Cylinder Head Set, an additional piston, Spring, and Spring Guide) and build two complete upper gearboxes, one an upgraded with the bore-up arrangement and one upgraded with the standard bore components.
If you only want the one upgraded upper gearbox then just order the bore-up components (4 Port Cylinder, Piston & Cylinder Head Set, Piston, Spring, and Spring Guide per the previous post.) and your done.
Because the Phillip head on the screws that are used to assemble the gearbox can get messed up when assembling and disassembling I would order an additional set of those because they are next to impossible to purchase locally: http://www.evike.com/product_info.php?products_id=34818