You have multiple issues that need to be addressed independently and then assess overall results.

First, go after the IAC issue. The IAC itself rarely goes bad. Chances are the electrical connector is loose or corroded. Pull the connector and use contact cleaner. If you adjusted the gas pedal linkage to compensate for this issue return it back to it's original position or else you'll never get it to work properly. Insure this issue is fixed before proceeding.

Clutch issue: If you are losing fluid mostly likely it's due to a leaking slave cylinder. On left hand drive Sportages it is located on the passenger side of the bellhousing. You'll see it. If that area is wet with brake fluid you found the problem.

Loss of power at speed: Agree with the group. One of the cats is clogged. Sportage cats are cheaply made and tend to break down internally which blocks the outlet. The question and difficulty is determining which one is bad since there are two on '99 - '02. Unfortunately it's trial and error.