I just flew back from Southern Utah and Colorado yesterday where we had daytime temps of 103F when we were there. As Vic said, the RV-10 has a lot of extra power in general. Certainly power and lift won't be the issue in Phoenix in an RV-10. Oil temps and CHT's may be. You'll have to be careful here.
On my recent trip my daughter landed at Leadville, CO. Pattern altitude is 10,727' MSL, with field elevation 9,933'. It was a warm day, so DA's were a couple thousand more at least. The RV-10 departed just fine with tons of runway remaining, and it was very neat to see the altimeter hit 10,000' within seconds of taking off. No issue with performance in the RV-10. We are very lucky as RV pilots that we can almost ignore performance charts. It's actually almost sad that it can make us poorer and more complacent pilots in that we have less to worry about and calculate.
For flying around Phoenix this week, I'd recommend basically not bothering doing it unless you have somewhere to go where it isn't as hot. It just won't be comfortable. If you do have to fly, do it in the a.m. You'll want to manage the flight by CHT and oil temp, adding forward speed in the climb. We climbed out at 125KTS to keep temps in check when we flew in the west the past few days. Altitude is your friend, so get there and stay there. LOP operation also is a great benefit to cooling. Just don't get yourself in a position around terrain where you are forced to climb quickly. That may cause you to mis-manage your engine.
Also, I'd have to say that after being in the 103F dry heat, I'd say it's still easier to tolerate than the 85-90F humid heat where we are. I got home and this week we're looking at 70's all week long. I wouldn't mind mixing some of our air with that down there so that being on the lake would be warmer.