I used to be into hydroponics a bit for orchids, and even attended a national hydroponics conference, where most of the talks were about tomatoes, and no one mentioned root temps. To me, that means optimal temps for the tops were good enough for the bottoms, and around 85 F is the generally the optimal high temp for greenhouses even for tropical plants. Above that (in the 90s) you start to get photo respiration etc. in the foliage. I expect that tomato roots might prefer it a bit lower (in the 70s). You could search around the internet for info on tomatoes and soil temps. I know that the is info on low temps for planting so their might be info on high temps as well.
Good luck, and let us know what you find if no one else aroud here has theinfo.