I'm considering adding a 100cm mast to my quiver.

This is because of riding conditions of large wind chop with very short period, I find I am breaching more often in these conditions and finding it challenging to gybe.
I can make the gybes but it takes more effort to do so.

I'm riding ocean swell & not using it for surfing, ie. not cranking had turns etc.

I'm wondering if the 100cm would help - it can ride deeper underwater and thus not be affected by the chop as much making the overall ride easier?

Any other pros or cons for the 100cm option...?
what am I missing...?

I'd stick with 85.. I have 75.. 85.. and 95 and I'm happier with the 85.
The deeper you run your foil there is noticeably more drag making it harder to get up onto the foil to start with and even while foiling unless you keep the foil close to the surface like you normally do.. Also depends on how big your foils are..
I'd try before you buy if possible..

I love my 1m the best definitely my favourite especially in a open sea

I enjoy my 1M Armstrong mast in the conditions you describe. Very nice to be able to glide over serious peaks and valleys without having to graze the top of them or ride them like a pump track.
The Tall mast also allows me to recover a breach before I touch down. Really cool feeling if stalling to be able to dive the foil and re-engage to continue pumping on a blown gybe etc.
Keep in mind that you may feel more flex than your shorter masts, as the loads are higher due to the increased leverage. If you are heavy, it will be magnified. If you are a strong rider, it won't be so much of an issue though, just need to keep driving the foil.

I have sailed about 6 with a 105cm axis mast, I am happy with the sensations but still learning. I really like the maneuvers and the rough sea but it is true that if you do not have the foil supported correctly, sometimes it feels a little pendulous and unstable

I ride a 95cms now for almost a year. Changed from 75 because of the inconvenience in the chop. Definitely best size for me, adding to that also a couple of extra seconds for gibing. Love it large chop (25+knots S-SW in Hampton) where I notice having less issues than others cutting across those big rollers and beat of all super fun riding it!
Although I have considered going bigger is the pain of entering the water since I ride a sinker, needing too much depth for water start.

I ride a 95cms now for almost a year. Changed from 75 because of the inconvenience in the chop. Definitely best size for me, adding to that also a couple of extra seconds for gibing. Love it large chop (25+knots S-SW in Hampton) where I notice having less issues than others cutting across those big rollers and beat of all super fun riding it!
Although I have considered going bigger is the pain of entering the water with my sinker, needing too much depth for water start.

I'm hearing Armstrong is soon to release a 93cm mast...I wonder if this is a better option than the 100...?