Just look at the graphics expansions... compare the Mongol Faces DLC and Shaytana's Better Looking Characters and decide for yourself which one would be a better product if they were both available on a retail shelf. The Mongol Faces are okay but there's not nearly the variety that BLC offers. Which should not be a surprise because Shaytana is an incredibly talented guy and he sunk an enormous amount of time into his mod. The Mongol Faces are okay too but for example the variety in faces is rather bland, and there is only a very limited number of clothes and props that come with the DLC. It's a product that the artists did on a scheduled time and that had to be done by a specific date and you can tell.
Converters would be the same thing... Paradox wouldn't have infinite time to tweak, say, the province-mapping or the weights on how tech levels get translated. Their product would likely be well done on a software engineering basis, and finished a lot faster than a fan made converter, but it would certainly not see the intense tweaking and fine-tuning that a fan effort sees.