My question wasn't in the context of making any records. I was asking to see if anyone has done it so that it's not just a guess and to get an idea of how much longer it might take.
Oops; ninja'd while typing.
Figuring it the way you mention is certainly one way. I suspect - due to gravitational effects allowing ships to go faster at times while inexplicably slowing them at others - that it's not as simple as adding 25% to the time achieved without using supercruise assist. I suspect that it would be longer than that. The star system's bodies are in constant motion as well, so unless you can start your trips with them in the same positions every time it's going to be pretty hard to predict, let alone compare, even simplifying to include just the few bodies that exert most of the influence in the system. If the bodies were aligned in a favorable way on that one trip that you decided to use SCA, who knows- maybe you'd get there faster. I'm not up on my gravitational simulation equations.
What can I say? I am curious.

When you begin a long journey like this, I suspect that your starting trajectory (relative to bodies and planes, etc.) has a pretty significant effect on how the whole trip goes. I personally have experienced peak speeds anywhere from approx 1500c to >1900c. I can't say that I have closely timed my trips, but - to the minute precision - I have seen times from 1h,23m to 1h,31m+.
btw: I AM on that list, just not with this name (long story that needn't be related here). That said, most of my trips to Hutton have been made while not trying to note or to improve my travel time. I really like that the HH site keeps a record of travel times. (I presume times are recorded only if logged-in or if using the Helper plugin.)
Thinking about this brings up another question regarding he times shown on the HH site:
Does the clock start as soon as you drop-in from your hyperspace jump? If I were to scoop fuel before actually heading towards H.O., for example, it would impact the time recorded. It would be ambiguous and challenging to start the clock another way in a consistent manner so I expect the answer is YES. I might have to resist my habit of scooping next time to see if I can best my lowest time.
Have fun trucking. Back to it for me too.