To answer your original question - No - a larger tube does not eliminate the need for a carb balance. I still balance my carbs at annual, and usually check it one other time during the year, if needed. I've had my rotax for three years now, and once I got it balanced, I really never have had an significant unbalance issue. Knock on wood I guess.
The larger tube is acting like a dynamic absorber between the carbs at low rpm's. It is absorbing pressure pulses between the cylinders at low speed. As noted, above 3000 rpm, the larger crossover tube has no effect.
The really nice part of the larger tube is that you can now idle your engine down to about 1350 or 1400 with no shake rattle, n roll.
And, if you install the larger crossover tube, don't let anyone tell you it will tear your gearbox apart. I have measured the vibration coming from the gearbox at 1350 rpm and it is .08 inches/second velocity - which is excellent! My engine currently is balanced to .03 in/sec at 5350 rpm, which is where I cruise.
As to the reason that Rotax doesn't change the crossover tube - who knows - believe it or not, that just may not be a significant issue for them. The simple fix is to tell all of us to idle at 1800-2000 rpm, and it kinda goes away. Problem solved. No cost to Rotax.
I have sat in engineering meetings where a supposed expert convinced our lead engineer to perform a certain procedure . Several of us on the staff tried to convince him not to do this, but our lead engineer had a rather enlarged ego. It ended up costing our company in excess of $300,000.00. No kidding. Look, these engineers are just people like you and me. Some really are smart, but some are incredibly dumb too. They should have stayed at the university and taught. LOL How many automotive recalls have you read about in your life?? Hey - those vehicles were built by engineers.
I'm not down on people who decide to become engineers. But, please, they are not infallible. And just because Rotax did not put a larger crossover tube on the engine does not mean that it coulld not benefit from one.
Sorry for the long post - I really hope this does address your original question.