Constant modulus algorithm - performance poor?
The constant modulus algorithm does not work for QAM because QAM is not a constant modulus signal (amplitude at the symbol decisions is not constant), such as BPSK and QPSK. 

SIR is the "Signal to Interference Ratio" and the greater the SIR the better the performance, not the other way around as you describe. With a higher SIR, the signal is stronger relative to the interference.