i think speed cut only applies to jdm models only. if u got ur half cut from japan including the ecu, then most probably it will have speed cut (car will feel like off throttle at certain speed ussually 180).
other japanese cars in malaysia dont have speed cut. thats why our meter shows more than 180. unlike jdm cars. even jdm impreza have only 180km/h max.
if ur car is using jdm engine and can go more than lets say 180.. most probably is that ur speed sensor behind the speedo meter is not functioning. therefor not reporting the actual speed to the ecu.. which is good :)
if ur car cant go more than the speed it is already in, that means uv reached the top speed for ur car. not a speed cut. mod ur car la to be more speedier. :)
about the removal of speed sensor brings bad things to idling speed, i dont know. all i know is my proton satria runs all this while with a busted speed sensor.. and it runs fine.. i know this when installing rsm recently.. coz the speed is not showing(on the rsm). later change to new speed sensor (10-20ringgit) then everything is up on the rsm.
just my opinion..