There is a reason almost all of the AA and fast A guys here are on 250s. They have the right kind and amount of power for all situations. 300s yank harder and over time wear you out faster. If your just a trail rider then sure they are great, but for racing here they are too much.
For more low end from that 250 you have a lot of options, and I have done it on several of them:
JD Blue needle
RB head mod
VForce reed block and '05+ manifold to replace RAD Valve(requires a little case matching)
verify ignition timing
high volume PV cover
verify correct PV adjustment
Gearing? 13/51 is good overall, 13/52 in the mountains
The '01 XC250 is a nice place to start because it has an aggressive CDI that gives good bottom and a strong mid rush. You should have the 2K-2 ignition as well. Flywheel weight DOES NOT increase your low end, don't get hung up on this. When everything is set up right the bike will be almost stall proof with no weight, but a 10 to 14 oz spin on works good in all conditions, and can be R&R in seconds. My 250s have plenty and lug down to just about zero, but don't pull on your arms as hard off idle and have less extra power to manage on that last tough lap, or last section of an enduro.