Before I bought my CTX, I test-rode a Moto Guzzi Norge, which in many respects would be similar to an ST1300 with the longitudinally-mounted engine mostly encased in plastic. I only ever got up to 50mph on a back road on maybe an 80F-degree day, and it was awfully hot. Like I had to spread my legs out before I made it more than a few miles.
Compared to that, the CTX is downright cool. I notice the heat a bit on hot days, but actually mostly when pushing the engine at higher RPMs, either engine braking downhill or accelerating aggressively in a lower gear.