Pretty standard upgrades to the stock Tigers are/used to be (from memory) :
-1- Downhaul going from non-cascading 1:8 to cascading 1:12
-2- Replacing all cordage and trim lines with swiftcord, dyneema and D12.
-3- Loose boom fitted mast rotation system. Replace any purchase system by a simple 1:2 and jam cleat system.
-4- Taper the mainsheet.
-5- Replacing the snuffer system. Even the newest Hobie snuffer is not well received overhere.
-6- Move about some of the cleats to personal preference. Most often moved was spinnaker halyard cleat.
-7- Often the single line spi halyard system was replace by a double line system.
I'm sure a few more mods will pop up in my head tomorrow, but this should get you started.
I've heard some claims that Hobie has been offer a Mitch Booth Tiger to serious European F18 sailors; this version was said to have most of the mods straight from the factory/dealers.
In the past all the sails would be replaced as well, BUT the new stock pentex STX suit of sails seem to be rather good. In the past hobie spinnakers were pretty mediocre but I don't hear people complain about them anymore so I guess these were improved as well.
Some replace the joysticks by much lighter ones, improves speed in manouvres.
Before the STX sails and Hobie offering modified Tigers directly it was said that an additional 3000 Euro's was needed to make a stock 15.000 Euro Tiger competitive in the top segment of the F18 class.
Personally I think we all must really do alot of on the water practising before thinking about mods and spending money on it. Pretty much my rule of thumb is :
If you are not consistantly finishing in the top 20 % of any fleet then no modification to your boat will make you more competitive. Only improving your skills will. From then onward, I regard mods that make your sailing and manouvres quicker and smoother to be more valuable then new sails etc.
I hope this helps.
Wouter