A bi-turbo is one of the popular performance exhaust systems.. it's an "expansion chamber". They do improve two-stroke performance withonly minor drawbacks.. one is they are loud as hell. Another is the bike is no longer "stock" and will require some tuning skills of various sorts to maintain the bike's reliability.
it does sound like the exhaust has carbon build up.. One of the Seafoam products has been known to work on carboned exhausts.. Since carbon doesn't dissolve, the burned fuel and oil residues and junk that holds the carbon together is attacked.. and it falls apart and can be flushed.
Search this forum for lots of threads about carbon and techniques for removing it.. it's not easy in the best of circumstances.. Also, put some effort in the direction of fine tuning your air:fuel mixture and perhaps select a new type of oil .. and maybe adjust the fuel:oil mix..
If you manage to get everything right the bike will run great and there will be adequate lubrication without a lot of carbon buildup.
That thing about soaking in gasoline followed by a worsening problem reminds me of something i once did..
Planning a long trip in an old car, i put a can of radiator rust remover/cleaner into the radiator. About 10 miles later the car overheated. The cleaner had loosened all the rust and that rust was then free to circulate.. and it formed a total clog in the water pumping system.
same thing may have happened to you.. the gas loosened and dislodged a bunch of junk, which was then free to move and get packed together in some narrow spot.