I have also made the move to an F18 from an i20.

I've only got 2 days sailing on my C2 so far. 0-5 on the windspeed.

I felt our downwind speed was OK, about what I was expecting.
you need to heat it up a bit more than the I-20 then work it down, can't just sail down like the I-20, unless it super super light then settle into low mode even if its slow. If in that low and slow mode and aren't even close to flying a hull pull your boats up about to the strap. Leave boards down to help facilitate flying a hull until the hull in popping up super aggressively that you can't control it with two crew members sitting on windward side."
We really struggled upwind, we lacked height and speed very badly. The boat seemed very sluggish in the puffs, and seemed very slow to accelerate.

My upwind sails are 2010 vintage, but I think they have been heavily used in that time. I have a new jib on order.
"new jib will help pointing a bit. we have 3 traveler car positions, just inside the inside post, centered overed inside post, and just outside the outside post (i dont know if weve actually used that last one). Generally we almost always sail just inside the inside post."
Our mast rake was at the top of the plastic clip above the lower pintle. Rig tension was 110lbs, diamonds were at 450lbs.
"mast rake is correct. Diamond tension 23-28 on loos gauge, tighter as it gets windier. Diamonds 37 base in super light air, 38 in anything over 5 especially if there is chop."
With a crew weight of 350, what should should I start with as a baseline on spreaders and diamonds?
"on 2010 sails they reccomend around 45mm on spreader rake, sam is right the new deeper sails use more spreader rake. If your light air and pointing is hurting try a bit more spreader rake. Personally I used 55 and liked it, tried 50, didn't like it, and went back to 55 and have been there since. We generally sail at around 390 but I sailed America's at 330 at the same. In reality the number is just a base number, you need to figure out the best setting for your sail and crew weight and it can vary largley."
I could be just expecting way too much coming off of an overpowered i20.