You havent suggested that its a boost hose leak mate, but some others have! you can take the boost hoses off a YB and the idle speed will not change!

your thermostat should not open till over 80degs, depending on type, and the car should be out of cold start by that time. thats the whole point of the stat.

i would suggest you have an air leak somewhere, becuae a petrol engine will not rev without air, so i would pressure test the inlet system (behind the throttle body) and investigated for air leaks, before having the fuelling evalutaed and the base idle adjusted to suit.

the intercooler or rad fitment shouldnt have caused the issue.

does the idle drop to its normal speed if you unplug the ISCV?