I have had both on different cars, my earlier car having the single sided and you are fully aware of the nice rumble and occasional pop but quite a muted experience, a 3.9 as well.
My present car came with the Librands Sport dual system but standard cast iron manifold, I can't say I have noticed any difference in performance but the sound is full on rumble with lots of popping on over run at around 1,500rpm. Ground clearance not an issue.
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Richard
The most restrictive part of the standard set up is the cast iron manifolds , the design is absolutely dreadful, just using the tubular exhaust manifolds will release about 15 bhp and a better torque curve.
You won't need to measure it on a dyno , believe me , you will notice it the very first time you arrive at a corner much faster than you expected.
The +8 tuning regime I would follow is
1 Better exhaust manifolds and decent silencers
2 Better intake system (K&N or foam type air cleaner to replace large box and trumpet
3 Tornado chip for ECU
Those will give you about 20 bhp . Mine went from 162 to 182 (measured on the same chassis dyno) , but the torque peak moved from 4,700 rpm down to 3,600 which meant the power was much more usable.
