That tank is not for overflow - it is for 'expansion', and an integral component, so again, without which the cooling system will not function correctly across heat/cool down cycles. Air in the cooling system will raise heck with the ECT, as one example, which leads to unreliable inputs the computer uses to decide air/fuel ratios.
There is only one thing behind the pass. side headlight that can make clicking noises - the IRCM. On an '89 SC, it controls the cooling fan speeds, the A/C clutch and EEC power.
Lots of things could make it run 'sluggish'. Vacuum leaks can manifest in many ways, depending on if they are small or large, and where they are. Do a visual on all the rubber lines you can spot and throw a wrench on the nuts for all the intercooler tube clamps.
Pull the codes to see what turns up there.
Might need to pull the spark plugs to be sure they survived the head gasket issue.
See if the fluid in the oil pan still resembles oil...coolant makes a lousy lube and might be the bearings/rings are trashed, dragging away power, leading to the 'sluggish' feeling.
Consider doing a compression test.
Check the brakes to see if they are dragging.
If auto trans, confirm the TV cable bushing is intact and the cable is adjusted properly.
Confirm the air filter is clean/clear.
Check fuel pressure at the rail...report what you find at idle, and under boost.
Confirm the throttle plate is opening all the way when the gas pedal is pressed to the floor.