Here's the thing with Logic and any other software-based music production software, for that matter: processor speed is almost always your greatest bottleneck. Hyper threading does enable twice as many virtual cpu channels in Logic, and Logic X is pretty efficient at distributing CPU load (but each instance of a software instrument/audio plug in is usually restricted to a single channel, hence multi-timbral instruments don't really benefit from multi-threaded distribution).
Bottom line: Hyper threading helps, clock speed helps more. For music production in general, the faster the CPU, the better. If you're serious about producing, do a BTO with the 4.0GHz i7. Not just for the hyper threading, but for the additional clock speed. You will get by with the i5, but it will show its age much faster, and given the nature of an all-in-one like the iMac coupled with the 5K screen, you're going to want to future-proof whatever you get. Also highly recommend 16GB RAM and at least a fusion drive, if you can't afford a full PCI-e SSD upgrade.