I never transit at 7am, although I do occasionally transit HKG. I sometimes exit and reenter for different reasons (usually to drop off or pick up physical items for my next flight), the end result is the same as what you propose.

Its efficiency is gate dependent. If you're dropped off at gates 1-25 or so, and your departure gate is a similarly low number, it could maybe make sense if the transit lines are as congested as you experienced. I've personally never experienced them that bad but I've also never experienced the 7am transit (at least not in a decade, well outside my memory window!).

The plans inefficiency breaks down in a few places:
*Arrival gate, which is fairly random, but CX seems to have nearly exclusive access to most of the 60s gates and some of the 40s meaning those gates get a high pct of flights and happen to be the most inconveniently located from immigration/security aka farthest (but closest to Pier and Bridge)
*Departure gate: same issue as above, special sting if your arrival and departure gate are 60+ for example, and trying what you suggest
*Departure security, I'm not sure if you get the CX priority security pass thing, I rarely navigate the regular line and it usually seems fine but occasionally I've noticed maybe a 5-10 min wait vs. usually instant for the priority line
*A not-small amount of potential walking, if you care

Overall I probably wouldn't recommend it, however I'll also add the exit transit / reenter strategy is extremely easy and very predictable. About as easy as it can get. Just I'd be surprised if it really was a time saver, unless perhaps you arrive at like gate 5 (next to immigration) or something like that, and the transit checkpoint there is backed up. I wouldn't do it to save time with a gate 40-60 arrival and departure, although as I mention above I've done the landside transit a few times before for other reasons including a unlucky gate pairs, and it's never a huge problem.