you can hear bad stories about almost anything, but this bad stories generally come from a bad marriage of components, exhaust , cylinder kit, gear.
The Scorpion exhaust is a very good exhaust and the RZ and SIP are very good exhausts too.
The bad stories heard about the scorpion comes from its power range response. The Malossi 210 kit it is at the base not that strong at low, mid-low and mid, then from mid high grows fast.
Scorpion it is the same thing. If you have a Malossi with 26 then you won’t have the same high rpm power then a modified reed and 30 carb. So when with a Scorpion you will hit its peak this peak will be lower and when you go from 3 to 4 you will find your engine in a rpm zone that the engine will find difficult to climb.
With a Malossi 210 reed and 30 in dyno test I generally find in the range from 5000 to 6500 a difference of more or less 2 hp in favour of SIP and RZ in comparison with scorpion. What scorpion looses in this range it gains after 6500.
The differences from RZ and SIP are in few words:
RZ right hand more low mid to mid-high, less high then SIP,
RZ 2000 left hand very similar to SIP just slitly better poower curve.
The SIP performance is avalable only in stainless version.
The RZ also in black.
This means that with your engine set up I will forget the Scorpion even though excellent for other kind of engine.
Hope this would help.