Talt to Dinomite Deisel in Monroe or to ... an outfit in Bellingham name excapes me right now..
That's DKDiesel in Bellingham on the corner of Bakerview and Hannegan.
DJ the owner and Jason the service writer are good people. Been friends of mine for quite a few years now.
They will sell you parts for your install too.
DJ knows is way around the Duramax's quite well.
He can also EFI Live your truck too and has a lot of programs for it for whatever level you desire.
He just EFI'd my 07 Dodge with a mild tune on Friday.

Aftermarket inj are typically cheaper than new from the dealer. The after's come new from Bosch and are then EDM'd or extruded honed (depending who you buy from) for better efficiency than stock. You will see mild power gains and better mpg's out of a slight increase in injector size.
No, you don't need to mod your CP3 or do a mild tune for slightly oversized injectors. The CP3 is capable of pushing enough fuel from the stock lift pump so you don't drain the fuel rail. Anything larger than the lowest after markets will drain your fuel rail on full throttle runs.