"Doug Roe's words capture
"Doug Roe's words capture the thought much better than mine but here's the thought process for rebuilding what you have:
GM supplied those engines and owned RPD at the time - RPD owned the Q-jet. GM's R&D center (test facilities) drawfs any marine OEM's similar centers, probably even in aggregate. Though the marine Q-jet is an "old" casting, all of its tuning parameters (rods, jets, bleed holes, springs, etc) can be optimized to suit a specific application. Who better to do this than the vertically integrated company who's reputation stands on the end result?
As far as "new" q-jets, there aren't many new RPD q-jets left. The closest would be the edelbrock-owned ones and they are all "newer" (APT-styled) castings. the point is these products come from a source without that huge R&D complex so the likelihood of them being tuned for your engine are small.
As far as the "remanufactured" products, the devil is in the details. For example, Mercruiser used several different casting numbers for big blocks and small blocks over the years. Look how many unique parts numbers a "remanufacturer" has available for a product line. I've never seen a reman house that offers as many carb numbers as what was factory original/available. Though there may be subtle differences between several castings, the point is one or two "reman'ed" part numbers can't match the total performance spanned by over 10+ different "OEM" part numbers - some compromises had to have been made.
Another benefit of rebuilding what you have is that you have the opportunity to know EXACTLY what is in that carb. Using this info, you can "tune" the carb to be uniquely application specific, if desired. Here's what I learned from Crusader's tech support lead long before they moved to SC...I asked how lean I could rejet the carb on the 454's that were "new" at the time. he said I couldn't - voids warranty. After a few more calls, the short story goes like this: "We can't control what hull the boat builder installs this engine in. We need to have reliable products. The reliability accomodates 'worst-case' conditions. We stand behind our products." My interpretation - "our carbs are jetted on the rich side to ensure we won't have a lot of warranty claims for melted pistons due to lean conditions. If your boat is light weight, you can probably lean the curve without problems." The first mod I made was to change the operating point of the power system. It was significant. My mods, in total, have reduced cruise consumption 25% over factory settings."