Something to consider is that the seals may have dried out while it was sitting unused and you are now getting an air leak at high power settings. The engine is drawing in air through the crankcase seals causing a lean mixture which you were able to compensate for with primer. At a lower power setting less outside air would bypass the carbs. Now that the seals have had some oil on them they're probably a bit better but as you found with a passenger high power settings are still a problem.
This is a common occurrence among two stroke engines that are not used regularly. UPAC has a thread on this http://www.upac.ca/forum/showthread.php?t=36
The cost to disassemble the engine and replace the seals isn't really expensive. I don't know how many hours on the engine to know if it's due for a rebuild anyway. I would probably want to consider replacing the rotary valve shaft and seal. Some coolants cause deposits on the shaft causing the seal to leak. Coolant gets into the rotary valve oil and the centre bearings causing them to self destruct. The rattling sound is really unnerving to your passenger (ask me how I know).
http://www.kodiakbs.com/ has some excellent PDF's that show the engine broken down by individual parts which will help you decide whether to tackle this yourself or have a Rotax shop do it. A couple special tools are required.
Some other things you might also want to check
1. New plugs - i replaced mine every 10 hours with NGK BR8ES or B8ES depending on what plug caps you use.
2. Did the carb sit with fuel and get gummed up. I don't think this is a common problem with Bing carbs.
3. I'm assuming that you left oil injection installed but was there any moisture in the oil or was the first batch gummy?
4. Is the rotary valve oil clear with no milkiness. By now if the rotary valve seal was leaking you should see some evidence of milky oil in the rotary valve reservoir.