If what it is worth most of the JDM aftermarket systems are 65mm for AE101's, I can't remember what brand i had on my 101 but it was not noisy at all, having said that there is a hell of a difference to a tuned and designed for the application type system and not something put together gungho at the local muffler shop with no knowledge!!
If i were you i would replace the section between where the extractors go from two to one thru to the first joint, there are some ugly compressions and restriction in that short piece! As for single or dual, dual looks cool but in reality unless you have a real tidy join you will get turbulance(back pressure) where the two rear mufflers branch off, with a single system you do not have this issue.