Hi,
U can try with increasing the length (2u or more) and avoid the cascoding.....but I don't know whether it will give a better speed....
again...if u don't stop the mirrored current MOS, instead u use a differential swith, i.e., when u try to push it to the o/p on the swith u employed and when off, u turn on another swith which will push the current to some dummy (like gnd/vdd etc.)....in this configuration u have to bother about only the switching speed of the mos.....and the magnitude of the current will not affected very much (but a little).....
sankudey