One of my instructors once told me, "if you wanna kick someone in the head so bad, kick them in the bladder first, then when they bend over screaming, the heads right THERE where you can kick it nice and easy, without falling on your ***"
:lol:
My feeling is, if you want to train to kick high for SD purposes, fine. But to be realistic about it, you have to take seriously the falling-on-your-*** hazard, and also the fact that you need to be able to create the necessary distance to get the kick
in. And both involve a different kind of training environment than people usually seek out.
For balance training in the extreme situations which a head-high kick involves in a streetfight, I think it would necessary to train on irregular, broken, sloping surfaces—the kind of real-world environments you're most likely going to be operating in when kick comes to shove in the big, bad real world. Let's face it, dojang floors are pretty artificial! And my own experiments with even moderately sloping surfaces has made it clear to me that achieving confident balance on them is a very different story from what I encounter in my TKD school. I personally would be very hesitant to throw a high kick—and I'm a pretty decent kicker, I think—if I hadn't practiced that same range of kicks on cracked asphalt driveways, broken pavement... the lot. And—in the same vein—on wet surfaces, gravelly surfaces (always a killer), soft ground... all the places where you can realistically expect to be finding yourself when things start going sideways. And let's not forget the clothing aspect—you better be training in the relatively tight jeans most of us have gotten used to wearing whenever we're not at work (or, in the academic world, when we
are at work as well

), in unsuitable footwear, etc. etc.
For distance-creation... practice in a deserted school classroom with a lot of desks, or your rec room after you've moved a bunch of tables, chairs and other hazards into it. Yes, interior spaces are more likely to have at least even floor levels; but they're also much more likely to be filled with junk of various kinds. To get enough distance to place a high kick, you need to add a foot or two to the in-your-face fighting range where you're likely to be confronted in a bar, club, lobby/elevator/etc. But you need to be able to do that in spite of the presence of furniture, bars, fixed counters... the works.
What I'm getting at is just this: the foregoing seems to me to be a reasonable, realistic picture of the training requirements you need to face up to if you want to be able to create high-kicking distances in any given real-world space, and administer that killer kick in stable balance. It's not simply a matter of practicing in your dojang in ideal empty-space/flat-floor conditions. I think this is a big reason why high SD kicks
are so risky: they have a lot of built-in hazards that probably very few people consistently train for...