Well, I think I can say that I'm fairly tech savvy. Basically the Core M (m3 up to m7) is always the same chip. However, after the manufacturing process Intel runs quality tests for each chip, and briefly said, picks the best CPU samples for the m7 line, because those can sustain slightly higher heat stress, and thus can be clocked higher.
Let's just assume for simplicity's sake; Intel were a road construction company. So bumpy roads get lower speed limits, while well paved roads become sort of German Autobahn. And that's how Intel basically does it its CPU pricing.
The m3 has a few 'bumpy roads', while m5 has much less, and the m7 has 'racing track' qualities. Back in the CPU world, it means that the m3 and m5 chips will be clocked lower and throttle their Turbo Boost compared the m7. The m7 behaves like a crazy race driver in a sports car. Driving for short bursts at max speed, before its motor overheats, then dropping back to cool down, but in the end it's faster for short sprints.
Due to the passive cooling solution constraints eventually all Core Ms will throttle back to the base frequency or even below in order to prevent physical damage to the chip.
If you however do rendering for hours, or converting videos etc. the m7 won't be that significantly faster compared to the m5, unless you cool it down externally. I remember how "Linus Tech" demonstrated that in a funny way by putting the last year's MacBook into a cold water bath.
I believe, the m7 only makes sense for people, who want to do light Photoshop work or simple raw photo editing, because that's the case where the additional 400 Hz at Turbo Boost (m5 vs. m7) can make the overall editing experience feel slightly snappier (e.g. adjusting brightness/contrast sliders etc.). For the web, office or mailing tasks even the Core m3 is perfectly fine.
Most people wouldn't notice any difference in speed if their MacBook was swapped out for a lower clocked model overnight. Maxed out CPUs are just phallic symbols of our modern time.
I'm concluding my post with Intel's raw spec spreadsheet for clarity though. Please note that Apple is natively
overclocking the base frequency on all Core m CPUs — like they did on all previous Broadwell MacBooks last year.