The MPA associated with the BSSO decreases only by about 1 degree (so there is a little CCW going on to get the extra jaw advancement) and the 'down and out' genio increases anterior vertical facial height. The BSSO advancement is pretty much along your native MPA and forward advancement is in direction of downward and outward. Although jutting out the jaw does not exactly mimic outcomes of a bsso, the direction when one juts out is down and out. So, relative to what you told the surgeon you wanted to see (more lower jaw advancement than originally offered with the 6mm advance), the proposal depicted on the skull model looks like a very good plan.
Your 'dilemma' now becomes one of being OFFERED the OPTION of having the extra lower jaw advancement you want (more than the single jaw surgery could offer) and deciding whether or not you would accept the possible trade-offs of changing the upper jaw to get the lower jaw advancement you want. Because there's NO option of getting the extra lower jaw advancement you want but not changing the upper jaw to do it.
The surgeon gave you 2 options:
First option was BSSO only (of 6mm?) + down and out genio which often involves removal of 2 lower jaw pre-molars.
Second option was to accommodate a request of MORE BSSO (lower jaw) advancement than that obtainable by lower jaw only. So, YES. If you want lower jaw advanced 8-10mm, then the surgeons's proposal accommodates that.
Ultimately, you (and of course other people too) want to be CERTAIN, that you will like the outcomes you see in the mirror. No one comes out and says that exactly. But that's what they want to know when asking questions about what the surgery will do for them specifically.
The deal is that no exact CERTAINTY can be had for anyone on here given the premise, (whether they state it outright or not), is they want to be certain they will like what they see in the mirror. Why? Because the very nature of choosing between 2 options is one of making a decision under UNCERTAINTY. That is especially so when 'risk aversion' is expressed with either option offered to you. For example, with option #1, you're risk averse to the possible outcome of not getting the all the lower jaw advancement you would like. But with option #2 that accommodates the the 'big enough' jaw advancement you wouldn't be getting with option #1, you become risk averse to possible unwanted outcome to the base of the nose region.
Now I can't give you a CERTAIN prediction of which Option with possible outcomes you are going to like seeing in the mirror more than the other one. But I CAN tell you HOW to make a decision under uncertainty when 'risk aversion' is expressed with either option.
Here's HOW to make a decision under uncertainty:
1: Realize there is NO option to have BOTH the desired 8-10mm lower jaw advancement in the absence of the upper jaw surgery.
2:RISK AVERSION: What aesthetic outcome or 'risk' are you LEAST averse to (least afraid of)?
Option 1:
Possible outcome:
Having 6mm lower jaw only, sacrificing 2 pre-molars to have it and then seeing in mirror the advancement (although it would be an improvement) falls short of the advancement you were hoping to see?
Option 2:
Possible outcome.
Having the 8-10 advancement of lower jaw that you want but also having changes to the philtral area that you might not like?
So, Option 1 vs. Option 2 resolves to one of not having the extent of advancement you wanted for the lower jaw vs. having the extent you wanted for the lower jaw but with possible TRADE OFF of (unwanted) philtral changes.
Now, those 2 options and possible outcomes don't preclude your being happy with the outcomes of EITHER. Like you could be happy with lower jaw only and you could be happy with more lower jaw advancement and find it was worth trade off a philtral change (which most likely would not be major since it's a modest upper jaw advancement) in order to increase the lower jaw advancement. But.....
Since you've expressed 2 'risk aversion' concerns with EACH option offered to you; that of option 1 possibly not being 'enough' lower jaw advance for you and option 2 being one of 'enough' lower jaw advancement for you but with trade-off of possible philtral changes that you MAY or MAY NOT be happy with in X-change for having the the 8-10 lower jaw advancement you want, the decision UNDER UNCERTAINTY that you need to make is which option has the possible outcome that you would be LEAST risk averse to. (Which one would you be least unhappy with based on a possible outcome you are more afraid of.)
Another way of saying this is to ask yourself: What part of your face (here the base of the nose area) would you NOT want to risk in any way to change at all and is the 'trade-off' of NOT getting all the lower jaw advancement you would like a 'risk' you would RATHER take in 'X-change' of NOT changing the upper jaw area at all?
That's the decision analysis under uncertainty that you need to make.