I've run the Shimzu's soft rear with the exotek chassis, the only difference is that I use the Tamiya rear rubber foam insert with the tires. It feels them out a bit better then what comes with the Shimzu's, so I believe gives better wear and traction.
Our track is low grip asphalt and we run early afternoon thru to the evening. I ran a set for 6-8min races and was really impressed with the wear rate..held up quite well. Air Temps in the mids 30's Deg Cel for the day.
Maybe for your temp range the Meduims might do the trick, perhaps try a set with the rubber tire foam insert.