The plastic part is not the problem sticking wise, often the actual shaft sticks in the iscv.
If it was ok before idle wise and it is now 1500 rpm, then this suggests the base idle is correct as it was before, unless of course it's come loose.
This leaves us air getting in through a stuck iscv or through a leak, as it needs the air to raise the revs over what the throttle body bypass gives air wise to set the base idle speed.
You could use a wood dowel to block the hole the end of the iscv goes into, this will ensure the only air it should be getting is via the bypass screw, some just use a big wadge of bluetak to temporarily block, if this then idles at the speed it used to, you know it's the iscv at fault.
The WS kit could conceivably give a tad higher rpm as it is making a more complete burn of the fuel, but wouldn't account for 500rpm increase, and would have done it right away.
tabetha