Advice on head gasket

I have a 2009 forester XT with just under 200,000 miles. My cars been overheating when pulling off the highway. Thermostat replaced 6 months ago. Coolant replaced twice in that time. I took it to the dealership and they told me I need to replace the head gasket for $4,200 and have the heads machined for $800. Would you authorize this repair. If not, why? If not, what else should I consider? Thanks fam.