I don't think generalities are easily applicable to boxwood.
I'd like to see pictures of it last September, coupled with more information about the length of your growing season, general pruning habits, and any other relevant info.
I feel like making an attempt to display a boxwood in the state you describe, at that time, may technically be impossible, or, if possible, not without a large risk to the continued health of the tree. I believe* September is an "off" time for you.
I believe, if it takes 5 years to figure out regular growth patterns of a tree, it will take another 5 or more to find the tweak in the pattern to display at an "off" time.
The Japanese have shows at times that best compliment the tree being displayed in it's regular pattern.
I don't think we take this into consideration enough.
*The more I think about that the more I think September may actually be the perfect time to display boxwood there.
New green growth on boxwood is especially distracting,
the size though, as compared to old growth, is even more distracting.
Both of those things can be found on real trees though, the even more distracting thing, that doesn't occur on full size trees, and only present on broadleaf evergreens and tropicals like ficus, is the "2ft" wide leaf at the base of a branch.
The horticultural point being, in order to safely display a boxwood in it's prime, you have to be aware that you will be removing or preventing both newer growth, and older growth. So depending on your regular care timeline, you may find you are removing a lot.
The key for me, is being able to remove that growth, without adverse future reactions.
We really must understand more in order to know how long of prep time is actually needed.
Note, I believe "club shows" ugh...can have less than perfect displays, but that doesn't mean they should be displayed in a less than prime as possible state because of this, if anything, it should be executed better to compensate for the less than perfect display, or "not quite there yet" state of trees acceptable in a club show.
A "You may not be able to put lipstick on a pig, but you damn well try", thing.
I would be sure to have all the basal old leaves gone ASAP.
Go through and find all your "proper nexts", good forks, that will compromise the final shown state.
Then prune to them during summer dormancy.
The resulting flush of growth should be the only thing on display. Remember those shoots will have basal leaves that should be removed.
Timing of this is crucial, which is why I would like to understand prior patterns it shows.
Actually, it's not the timing that is crucial, it is the understanding of what else has to be done, depending on timing, that will have it in it's primers state.
On one hand, cutting back too early during summer dormancy, may make it push fall growth too early, which may become leggy with big leaves.
Too late after fall growth begins, and you will have an odd bit of ramification, with more basal leaves that would require removal.
Somewhere in between the 2 should work.
Doing parts of the tree early and parts later may work even better.
Sorce