Migration from SQLProvider to Custom Provider

Hi, 
  Quick question about Profiles. As the membership system only has a few categories, I'd like to add a few custom fields such as First Name, Surname etc...  I've decided that instead of writing to my own table, I'll use Profiles. I've heard that, due to the way data is stored, profiles become inefficient with a relatively large user base. How hard is it to migrate an existing Profile table to a custom one in the future? If I get enough users, I'd like to roll my own custom provider, but will it be easy to switch between the two, or are thre pitfalls I should watch out for?
(In the interest of time, I'd prefer not to roll a custom provider right now)

Thanks


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