Keep the car out of boost !!!
The OEM map in the ECU is flawed at best with 91/93. If you like your XT and plan to keep it and do any hard driving, get it to a Tuner.
Now if you had Cobb AP or the cables for a laptop, and had a Tuner make you a 87 map I would think you could use that. But I'm no expert in going to 87 octane.
The best idea I've read here is, rent a car.
174,000+ miles, 5/20/12 new ej257 short block, 8/20/11 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. Also CHECK YOUR OIL, these cars use it.