if you leave the splash shield off from underneath the car would that affect cooling at high speeds??

Not with a '99 Forester.

Not a good idea in wet weather.

When i bought the car it did not have one on it. The car runs fine but if you cruise over 65 it starts to heat up. Have replaced the thermostat, bled the system using that bleeder tank, replaced the antifreeze again and tried a another new thermostat. Its OK as long as you don't go over 65 for a period of time. ???

Check for coolant loss first, indicating possible head gasket seepage, which is extremely common for the SOHC motor. Check to see if radiator front fins are occluded with debris. Then check rear coolant hoses for blockage. Are you SURE your new t- stat is an OE (large orifice one? If not, it won't allow full flow at highway speed. If all flow is ok and you're still overheating have percolation test done (or easily do it yourself staring at a funnel squeezed onto the radiator top hole while you rev up a hot motor gingerly (be careful of splashing). If not sure, have wrench sniff overflow tank when performing same test for HC in exhaust bubbling. If yes, you have high pressure INTERNAL head gasket leakage, requiring a $1500-1900 repair, which is not advised with Subie this old. But first do the easy stuff...although the odds are indeed against you (sorry).