I think of myself as very well educated 
Yeah, but probably not Sg 78 well educated.
To give an example from my own collection, Inquisitor Lyra Rhodes is 98 years old. Been an Inquisitor for 41 years, was an acolyte for 22 years before that, and earned herself a Doctorate prior to being taken into Inquisitorial service. If IQ tests existed in the 41st millennium, she'd earn herself something in the range of 160. In short, she's well blessed with education, experience and intelligence.
For that list of qualifications, she gets Sg 81*. Bit higher than 78 of course, but it gives you an idea of what I take Sg values of around 80 to mean.
*Before anyone comments, yes, it has been changed since I posted her profile in December.Of course, if you near meet those criteria, then that's fair enough, but to me, Sg 78 represents a combination of smarts and schooling that isn't going to come around very often in our world.
On a more serious note, one of the theories I've heard regarding self-imports is that in the 40kverse, one must remember several things
... again, he may not be more than slightly badass (or even that unusual) by 40k standards. But compared to the real you, he almost certainly is more of a badass.
Evidently you want to interpret "badass version of yourself" differently to how I do. Whatever you think it means, what it means to me is "semi-humourous shorthand to say more skilled than the person it's based on".
I personally tend to avoid naming specific ages simply because it's such a stretch for us as gamers to connect with the characters we create. Sure 60 may be the new 20, but when you want a youngish Inquisitor (let's use Covenant as an example) it is incredibly difficult to envision an Inquisitor who should by all rights be in the later part of his first century as relatively young.
I wouldn't say that's really a problem. They just have a different cultural perception of age. Doesn't mean you can't put numbers on it, just means you'll need to use bigger numbers.
To them, the above mentioned Lyra Rhodes' age of 98 probably means about the same as an age of ~38 would mean to an average westerner... not that she necessarily acts it all the time! Obviously, she's much more learned and mature than someone in their late thirties, but that's merely how she's perceived in their world.
On a slight aside, I think the fact that rejuvenat can keep people physically in their early adulthood probably contributes to people staying more "youthful" in how they act. You act the age you feel, not necessarily the age you are, and if your body stays young, you're more likely to feel young. Which is why 100 year old Inquisitors don't act like your average centenarian - aside from the fact they're not hampered by an aged and frail body, they simply don't feel that old.Though this should be no means be abused, I think the simple answer to the question of "why do it that way" is because not all Inquisitors are created equal. There are endless instances I've seen in which managerial decisions are made that go contrary to traditional common sense.
Yeah, some Inquisitors will make very questionable decisions. However, what I'm referring to is an Inquisitor who actually possesses the trait of common sense. While there are certainly Inquisitors who make stupid decisions and choices, I think most would take at least a moderately sensible approach to recruitment of their students.
...so he spends a lot of his time going round Scola Progeni and other sources of talent, and recruiting the best of the best as Acolytes.
Not quite the same as "no assessment". If you're recruiting from the Schola Progenium, you've got a whole heap of records and reports to work from. You'd probably want to at least corroborate that in person, but a decade or more of paperwork is hardly a bad basis to use for such a decision.