I would go with the flat top piston and head mod, it works incredibly well on a 250. As Motorrad said, have the suspension done, I adjust mine by millimeters of oil height, usually the range is something like 110-130 mm and to accomplish this would be probably an ounce or two. the newer forks are tyically done by measurement, most of the older forks have both a volume and a measurement, I always did the measurement.
Of all the things you can do to make your bike faster, suspension is the best. you can have all the HP in the world but if you can't ride it, it's wasted power