Ok , here's my pennies worth ...
First , on a 4 stroker , a wider , larger exhaust opening ( no muffler , no pipe ) , is effective in overall top end performance , allowing both higher rpm range and hp .

On a 2 stroker , a tuned pipe , utilizes the pressure waves created by the exhaust gases to aid in the scavenging of those gases to create a "supercharger" , if you will . A tuned pipe will pull the fresh fuel into the cylinder better while also making sure that there is a minimal amount being kicked back out the exhaust before the fuel ignites . Now , with this , you will get a larger than normal ( or should I say , more than an open exhaust ) rate of fuel and compression , and hence , more power ..[8D] Now , how this "extra" power is used , is decided by which tue pipe and header length you use . It could be user for a higher top end , wider RPM power band , ability to swing a larger prop . Some other noticable attributes are : less wasted fuel ( meaning that less is kicked out the exhaust and basically just wasted ) , a cleaner burning/running engine , better throttle response and last but not least , one that has been bugging folks a LONG time which has so had many turned away from our local ponds and ponds closed due to this factor - LESS noise .
Now , I am not condoning, or condemming the use of a wide open exhaust , just stating that there have been MANY , MANY complaints over the years that has caused MANY ponds and other areas for RC'ers to be able to go and enjoy their hobby .
As a matter of fact , as much as I loath the "clubs" for boating ( NAMBA , IMBPA and the such ) because of their "rules" , I think everyone should also know that they have incorporated a "rule" which not only makes sense , but is " neighborhood" friendly - I believe they have lowered the DB for the 50 ft distance to 85 db's .
FTR - I own a BIG cat , with twin Zen's , both have QD WTC pipes . Now , what is my reason for telling you this ? Well , you guessed it ... I'll tell ya .
When I fire up those twin Zens , the sound is sooooooooooo sweet .... but ( yeah , you knew that was coming )
Talk about LOUD ???? The lake is off a kinda major highway ( for the area anyways .. LOL ) and I actually had a passer by , stop, and was watching and after I brought the boat in , the fellow ( who also has his wife and kids with him ) said to me ... shoot , we heard the " sounds" ( he was being nice and not saying NOISE ) from the road and thought that they were having some kinda boat races here ....
Then he went on to say that he was REALLY surprised to see that it was only from one "little" boat ( little meaning that he was initially referring to a FULL size , REAL boat [&o] )
So , to any who plans on going NO exhaust , please be considerate of the "neighbors" .
( man , I hate preaching ... )
But it is a fact ... most ALL areas have " noise" level laws , and besides, you also don't want , or need the local PD hassling you , OR worse , forbidding you , from running on your local pond, or where-ever .
So , in cutting to the chase .... make your decision wisely , otherwise , you might not have a decision to make for future use . ( meaning , you WON'T be allowed to run your RC's because of the noise levels )