Few questions there, can help you with the first one. There is a tip you can buy for the 2bro that will lower the dB a couple. Its not a lot but it may make it less annoying.
Power tip...
http://www.twobros.com/cgi-bin/shopper?search=action&sku=005-20804&searchstart=0&template=Templates/TBRSearchResult-Simple-Kaw-Ninja250R-SO-08.html
As for other exhausts, the longer the muffler the deeper and generally quieter it will be. You could go a quiet core AreaP full exhaust. Modifying the stock exhaust is another option. I would imagine the yoshi will not be that different to the 2bro, little quieter but its nowhere near stock.
Felix
Power tip...
http://www.twobros.com/cgi-bin/shopper?search=action&sku=005-20804&searchstart=0&template=Templates/TBRSearchResult-Simple-Kaw-Ninja250R-SO-08.html
As for other exhausts, the longer the muffler the deeper and generally quieter it will be. You could go a quiet core AreaP full exhaust. Modifying the stock exhaust is another option. I would imagine the yoshi will not be that different to the 2bro, little quieter but its nowhere near stock.
Felix