I feel your pain on the jib size. The largest jib that will fit on my Beneteau 311 is a 116% jib (the jib leads are inside the shrouds). On light air days I am at a significant disadvantage to those boats with a 155% jib, which is most of them. The PHRF handicap system we use here assumes boats will have a 155% jib. No credit is given for smaller jibs, unless a special one design class is set up. Once the wind gets up to about 8 knots then I can keep up upwind, but I still need more downwind.
My last boat had older OEM sails that were in good physical condition, but had lost much of their shape by the time I bought the boat. They were much more rounded than new sails, so when a gust hit, the boat just heeled over more rather than accelerate. When I got new sails, it was like a different boat. When a gust hit, or stronger wind, the boat would go faster instead of just leaning over.
Take a look and see where the deepest draft is in you sails. If the deepest part is in the middle rather than towards the luff (front to back, not up and down), the sails are too round and blown out. You may be able to get them recut to get much of the shape back. If they are too worn or too damaged from the sun, such that the sail will come apart if the cloth is restitched, then new sails are the only answer. Good used sails can sometimes be found online, or better yet, from a serious racer who buys new sails every year.
Good luck. It takes time to learn a new boat, particularly if you don't have a sister ship to sail against. Racing will accelerate your learning process. If you find something really seems to be working (or the opposite), take some notes afterward and maybe make some marks on the lines or next to where the gear was set. In time you'll build up your own tuning guide.
Time sailing in the boat is a big help.
On short courses a good start can be more important than anything else if you don't make any big mistakes. Do a few practice starts when you have some extra time, and get out to the race course early to see what's up with the wind that day and figure out a plan. You don't have to have the best start, and it can be better to start a little away from the serious sailors to get clean air and a chance to do your thing (assuming the line is reasonably straight into the wind). Sailing shorthanded, it's more important to be sailing in clear air, moving well, and in good position a minute after the start, not at the start. Once you're ahead of a least a few boats, stay between the mark and the boats behind you, loosely covering if you have a cushion, and go after the next one or two ahead.
