I've tried a lot of different muffler (stock, de-baffled stock, Borla, Dansk 84mm Sport, Bisimoto, M&K and some others I've forgotten about) and pre-muffler / cat / declawed cat permutations...
I found a SW chip + Dansk SS pre-muffler + Dansk SS 84mm sport muffler + stock airbox / filter with an open cover to be the easiest, best all around setup for performance and neighbor-friendly street sound. Monty muffler sounds GREAT too, but pricier and harder to source vs. Dansk Sport.
Steve tuned my '88 with the above setup on the dyno. It sounded good before, but it sounded amazing nine pulls later. Hearing the improvement as he tuned it between runs was eye (and ear) opening. 20 hp increase to boot. AMAZING. It was one of the happiest days of my life as I proceeded to drive my relatively new, to me, 911 from LA to Santa Barbara to SF up Hwy 1. Pure magic. :-)
My '89 has B&B long tube headers and a 2 in / 1 out muffler. I replaced a Superchips chip with Steve's off the shelf chip for a 3.2 with headers. Noticeable improvement in smoothness and drivability as well as sound.
It's laughable how much of a no-brainer these mods are.