I just got the X1 Yoga (i5 7300u, 16GB) and so far haven't had any thermal issues with it.
Performance section: "The comparison of the two Lenovo ThinkPad X1 Carbon 2017 devices shows that the Intel Core i5-7200U can keep up very well with the Intel Core i7-7500U during continuous load. The reason can be found quickly, since the temperature goal of the Core i7 is already reached after a few seconds, so that the performance has to be throttled considerably. Thus the advantage of the supposedly much faster processor drops to a minimum"
Edit: It appears you can pay more to downgrade to an i5 on the high DPI versions.
The main application where I expect to care about sustained load on such a machine is watching movies, and even then I assume modern systems have good enough hardware acceleration that this isn't much of a load.