As an XXL rider (if you are XL), I have a lot of thoughts! Keep in mind, if you ask 10 foilers their opinions, you will get 10 different answers!
First up, Hdip is right, mast stiffness matters…
Many previous generation masts – yes including carbon – were way too wobbly for you and me (particularly with wide wingspan wings like you will use for downwind). I think the current generation masts marketed as “stiff” or “heavy duty” or whatever are almost there. (I’m 6’4" and in the 230s.)
I believe that the reason Lift didn’t put out any foils with a wingspan wider than 39 inches is because Lift knows that the previous generation mast (which yours probably is?) is too wobbly to handle wider wingspans and bigger riders. (I have no proof of that!) It’s not a complaint about Lift. Armstrong’s previous generation wobbled for me, as did other carbon masts. I was frustrated. It seemed like AXIS’s cheap aluminum 19mm was beating what was supposed to be “good”.
I found that Cedrus masts were the ticket for me to use any gear I wanted (including Lift!). Some say Cedrus masts are too thick (at 19mm) and therefore the top end speed suffers. But at my XXL size, the rigid direct feel outweighs any potential top-end-speed disadvantage. Essentially, all of my pumping effort is delivered directly to the foil. I can pump out to sea with confidence. No worries.
A huge plus with Cedrus is, you can use any brand of foil and you don’t have to spend another grand or more on their carbon mast. Just buy an adapter for $120. Not a sales pitch for Cedrus. Just getting to the point:
Rigid is good, particularly for masts for big dudes.
Your Lift 170HA and the 28" mast is probably a good size for you (though I don’t know your conditions). Nice work on doing your homework, or kudos to the person who suggested that size.
Another thought though to make your life easier… I don’t know Lift foils particularly well, but you mentioned the 25 stabilizer… That sounds small to me, particularly for a newer prone foiler that’s a big guy. For stabilizers I tend to almost always run a size or two bigger than my bros… particularly in prone. I don’t mind a slightly bigger stabilizer when winging as well.
The simplest rule of thumb for stabilizers is… are you stable? Then try a smaller stabilizer (for more speed and maneuverability). Are you unstable? Then try a bigger stabilizer.
So… Are you stable? Or are you unstable?
A slightly bigger stabilizer allows me to pump back out with more authority… It might theoretically be slower. But if I’m less stable, then I’m less able to pump as efficiently.
Can you try a bigger stabilizer? The difference will surprise you.
On downwind SUP foiling, I don’t have too much to add. I know you are an experienced downwind SUP man. I’ve done Maliko runs and the Molokai race (as a team) on SUP. My limited downwind foil experience says to go for more volume than you think… like 20+ or 25+ over weight in kg. The most basic starting goal is to get the board unstuck from the water. To learn, starting with a board that is already “mostly unstuck” (meaning, with more volume) seems to make sense… particularly from my limited experience of really struggling to get it unstuck. That advice could be wrong! But I couldn’t get the board unstuck that I tried below that recommended size. Frustrating! I look forward to trying again, on a higher volume board.
Maybe reach out to Robert Stehlik of Blue Planet Surf for downwind board size advice. He is a super nice guy, probably your height and weight, and experienced at downwinding SUP foiling.
Man I have too much advice for XL (and XXL!) foil bruddahs.
Hopefully that’s a start!
The biggest thing is, be aware that what works for 150-pound teamriders isn’t always what works for 200-pounders.