Is this a good solution? Depends. What's your spark plug look like? If it's black and oily like the Exxon Valdez took a spill inside your engine, then yes, downjetting will probably help your performance.
As for the carb, there are plenty of other options to increase your speed besides just getting a bigger carb. You can change your sprockets (gearing) to increase speed, you can increase air flow with a high flow air filter (but be sure to make sure you've got enough extra fuel to go with your extra air), you can get a performance exhaust... there are a lot of options.
You can check Bing carb numbers here to help verify your carb size, and you can read about your speed-enhancing options here. break> Old posts are also a cornucopia of information; just use the search function and you should find enough information to keep you busy for the next month.