do what you want, its your bux - just remember this.....
straight pipes have two big issues
1 - if its just right the exh valves will get burned - this is not covered under anybodys warranty - valve job at best, new heads at worst. been there done that.
2 - open pipes may possibly give you really big hp numbers at 6200 rpm.. but at 2000 rpm you will lose something like 20 hp and 30 or 40 ft lb of torque.
a very large muffler like a stock can with a fulsac core is just about ideal - everything else is either: looks!, or look at me!, or ba** busting!. real big power at speeds under 5000 rpm in high gear is actually pretty quiet. 7000 rpm on the drag strip is not quiet.
its been a time honored deal that guys like really loud barking pipes - but if you go too far you go very far backward in actual power. its real real easy to get it to sound way cool with all the usefulness of a drooping d*&^ - when i was about 20 i figured out that the girls liked the noise for about 5 mins - then it had to be actually scary fast - and then the unit had better work. ahhh 79 sporster with the giant ugly exh and the speeding ticket on 35 north of des moines for 110+ two up. stone stock, big volume. [the girl wore heavy black leather with nothing else
i dont care how ugly the exh was, or what the ticket cost, that was worth it!]
obviously the other issue is volume - the exhaust pipe needs to be at LEAST 10 times the volume of the cylinder / cylinders it empties to work properly. the original reason that the ultra 110's have the 4" mufflers is to get that nessesary volume. other rule of thumb is that the outlet of the muffler should be near the same size as the exh port in the head - if its larger you risk reversion up to the valve and very costly things happening.