I have been told that once you replace the head gasket on this car it never runs as well again. Can this be true, as it only has 100K miles on it?
That’a not true at all but there’s a catch. It will run just as well as new IF properly repaired and IF there are no other mechanical faults that were caused by a leaking head gasket. (depending on whether the leak is simply an external one or a more serious internal one)
Internal head gasket leaks can cause piston ring wear, dilution of the engine oil leading to crankshaft bearing, camshaft wear, etc.