haha glad you like the JDM packing peanuts. :p
i'm glad everything got there fine and dandy. if you want a silencer i think you can find a generic silencer on ebay. just measure the exhaust outlet diameter. there are holes for screwing in the silencer on the exhaust already.
your speedometer will be off. you will be going slower than what your speedometer tells you. also you will put on more milage on your car even though you didn't actually travel those amount of miles.
just make sure the tires won't be too small.
http://www.miata.net/garage/tirecalc.html
use that site to calculate if the tires are going to be too small or not.