I am taking it for granted that the engine we are taking about is a stock '78 Euro 930.
My muffler is stock Euro. I've tried a half dozen mufflers and keep going back to stock due to noise levels. The waste gate however has a dump pipe. The result is quiet running under normal conditions and daaaaam loud when you want it to be. I love it.
If you want a few hundred RPM reduction in spoolup time go for a straight-through muffler design like Craig's. The turbo sound is an added bonus. 
I will add that headers become necessary when/if you decide to go for big power numbers.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
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