Do you have to use premium gas in the turbo?

What is your question?

I have the 2.5 L Turbo version of this model. Recently, it has been running very sluggish. I just had an oil change 500 miles ago, and I alternate between 89/91 grade oil per tank fill up. The manual states that higher grade gas is not a requirement for engine reliability. However, the way it is driving now, it's almost like it isn't a turbo at all. I made an appointment at the Volvo dealership here in Vegas. I see exactly what is going to happen. There will be something wrong with the transmission, and they will say it is because I am not using 91 gas. They already tried to tell me over the phone that 91 was REQUIREMENT for the engine, not a suggestion from the manufacturer for better performance. Now it could also be because it's very hot right now in Vegas and I know that switching to a higher octane in hot weather can sometimes help. Anyone else having this issue?