Looks like you're going to have a pretty cool build project. You're looking for opinions, so I figured I go through some of your questions and build components.
1) As far as boring the cylinders goes, I think the largest you can go before the cases need to be machined for piston clearance is +1.5mm. Now that being said, you're aren't going to get any noticeable performance gains from boring to a larger piston. I would keep the piston size as close to stock as possible for that reason, and also for head gasket sealing. The larger the bore size you have, the narrower the sealing surface becomes between the cylinder and the water jackets around the cylinder. I've heard of quite a few guys having head gasket problems when the piston size starts to get too high.
2) Wiseco pistons are good pistons, don't get me wrong. however, because they are forged pistons, you have to be careful you give them plenty of time to warm up before doing other serious riding. Wiseco pistons have a big reputation to giving cold seizures.
- also, depending on whatever pistons you choose, make sure whoever does the machining accounts for enough piston clearance in the bore. Most machinists stick around .005", but that little of a gap in a piston port 550 cylinder with cast steel cylinder can also lead to a top end seizure. Most machinists are surprised, but cylinder to piston clearance should be around .007" for cast pistons (this doesn't include Wiseco, they are forged). I think Wiseco recommends to run them around .005", but you have to give adequate time for warm up or you'll be seizing the top end.
3) High compression head: unless you want the hassle and have the extra money to put race fuel into your ski, I wouldn't go over 180 to 185 psi. I personally would stick around the 175 mark just to be safe.
4) Impeller: all comes down to personal preference. My favourite all around impeller is the Solas KB-SC-A 13/18. It gives a good mix of power delivery throughout the power band. Aside from that I would stick to a 15* or 15.5* straight pitch impeller. If you are going straight pitch, steer clear of the Jetlyne impellers, my buddy had one that never seemed to stop cavitating. Stick to either a Solas or Skat Trak
5) Over-sized steering nozzle: if you're going to buy this, it's nothing worthwhile. What you would want is an over-sized reduction nozzle for the pump. You can buy one of those, or bore your stock exit nozzle at a machine shop. I think a stock exit nozzle on a 550 is around 65 or 66 mm. You can safely bore it out to 68mm. This will help with bottom end thrust.
6) SBN44: a 'must' to get your motor tuned in well. The old round body Mikuni is okay for a stock boat, but nothing else unless you do some internal carb mods.
7) Ocean Pro Ride Plate: I had one on my old 550 and loved it. It as always my favourite
8) Jetting: On my 550 with stock porting and a Coffman Climax 1/2 pipe with an aftermarket flame arrestor, I was running 100 pilot, 120 high at 15-16 psi pop-off pressure.
***If you want a lot more information, check out PWCToday.com in the Vintage Ski section of the forum. There is a wealth of knowledge around there for the old 550s. That's where I learned everything. If you go into the Vintage Ski part of the forum, there is a link at the top called the "440/550 Bible". It has a tonne of info on modding 550s. Everything right down to different porting specs.
You will find this site very limited for info on 550s compared to the other forum.
