Transition to Larger Flue - Does it Matter?

[Mistertoy]

I am building a 35 inch pompeii oven and have been working on installing a 6" flue.
The transition is finished and it reduces to a little under 6" x 6". The ratio of door area to flue area is about 14%

But... I have just seen a good condition stainless steel flue that is 9" diameter

I am confident I can install the bigger flue over the 6" x 6" square hole but I am just concerned that it may not draw well with it going down to 6"x6" and then expanding to 9"dia.

Can anyone give me some advise if it will be a problem...

Thanks
Brett