I can't speak for CPIs or Shearers, only from talking to others about them, that the shearer pipes will not have any bottom with tons of top. the CPI pipes are better for all round than the Shearer and last but not least most "drama" surrounded infamous Rockets.....You will have several people talk shit / haters of them and you will have several lovers of them, me being one of the lovers.
I have noticed no loss of any bottom end with them, the mid range comes on very hard and they pull like a mo - fo on top, probably not as high as the Shearer, but with better all round which is a little more needed for round the yard and what not. My experiences with them so far are on a stock motor.
You can get some other reviews on the Rockets from these members:
"Loco"
"StClark"
"Savage420"
There maybe some more, not for sure right off hand. The biggest down fall of the Rockets is the fuel petcock is tricky to turn and is nearly impossible if the pipes are hot, the spark plugs take a little reaching around the pipes to get out and are not that bad, but again the pipes need to be cool. The Rockets however, are built with heavy gauge materials which is nice.
Mike