I've tried several of the RJS kits, including the 18L En Primeur. I assume that's the one you're referring to and not the 23L Premier Cru sterile must, right?
If you'd really like to see proof of the quality of the 18L kits compared to, say, a 9L-10L kit by the same manufacturer, look at the awards the wines have taken. Yes, the 18L kits are indeed almost always better than a "smaller" kit by the same manufacturer because the additional volume is wine concentrate and not simply added water.