Wing? Parawing?
Floater or sinker board?
What conditions do you see in your local area?

If you already have these, then you should be able to answer your question. Try the 3.5 on days you would normally go out on the 3.

Personally, I have one 3.5m wing that covers me for any conditions. Above 35 it can be a bit squirrelly for getting up, but I don't have these conditions often enough to carry a small wing (I used to have a 2.5m and got rid of it because I didn't use it).

Im 87Kg on an 85L ML and for higher wind strengths I have a 2.8 and 3.5 (different brands) and there is a noticeable difference in these. The 2.8 comes out when its really ON and gusty as its just easier to manage than the 3.5 and its usually windy enough that Im not trying to pump the wing to get on foil, its more about trying to hold the wing steady and pumping the board to get going/release. I also find that a bigger foil with more glide (CODE 860R or 680R ) is easier in super gusty conditions, as I can then focus on modulating my speed with the handwing instead of trying to keep a smaller, reactive foil under control as I get hit with big wind speed changes, wind driven chop etc

I wing in the Columbia Gorge and have 5-4-3-2.5. I make it work but so often think how much I’d enjoy a 3.5 as I’m riding somewhat overpowered on my 4. Haven’t pulled the trigger (yet).

My wing quiver is:
2m Strike v3
2.5m Ocean Rodeo Glide A
2.5m Ocean Rodeo Glide AA
3m Ocean Rodeo Glide AA
as an 86kg rider on a 68l or 77l midlength.

If you are looking to milk every last ounce of performance out of every single session then yes, having every size is great. I really don't think it's necessary though. Once you are riding 3.5m and smaller the gains of dropping a little bit of fabric here and there don't seem as valuable to me. If I was in your shoes I'd keep the sails you have and pick up a smaller foil for days when it's getting extra windy. That way you get the double benefit of if you trash your 3.5m one day you can probably force the 3m to work while you get a repair or replacement whereas a 2.5m will likely not save you if your 3.5 is dead.

WindMode said..
I used to use a 5m for all conditions. Now switched to a 4.5m. Im 100kgs. I think you'll manage with either. 4.5m gets me 12-35+ knots of range.

with the same front wing?

I’m 75kg and have the exact same quiver as you. I find value in having the 3m.
Percentage wise it would be a big jump from 3.5 to 2.5.
Tony