I've had a few different bikes ranging up to 750cc, and I have to say I love my 350F. Getting more performance out of the engine would be nice, but it costs too many dollars for each horsepower you scare up. Instead, I am focusing on handling. Bronze swingarm bushings, tapered steering head bearings, good tires (Avon Road Riders), forks from a 400F or 360T with new oil, rebuild/replace the rear shocks and go with lighter weight springs designed for my body weight (c'mon, like you are going to get up any hills with a passenger on the back of this bike), and maybe lighter rims down the road. My bike is very fun to ride with the tires, swing-arm bushings, and steering head bearings. I'm really looking forward to the other mods.
As far as power goes, I will probably mill the head down by 0.010", lap the valves, deshroud the valves if necessary, and do some minor porting/cleaning with the exhaust and intake ports, match up the carb boots to the head, and paint it a "go fast" color. That will open up a bit of power, but not much. I doubt I will go with pod filters, although the Kerker 4-1 I found will probably get bolted on after the head work.
That is all I'm interested in. It just doesn't seem worth it to dump loads of money into a 350cc engine to make more power. Smooth engine operation and lighter components are always worth it, but there just isn't much HP available on these without a lot of cash.
Camelman