If in 2010 your basis was $5,200, you made a $15,000 Roth conversion, you had a nonzero traditional IRA balance at the end of 2010 and your tax return reported a taxable amount of only $9,800, you filed in incorrect tax return for 2010. Because you had a nonzero balance in traditional IRAs at the end of 2010, only a portion of your basis should have been applied to reduce the taxable amount of the conversion. Examine your 2010 Form 8606 to see where the error was made, perhaps by not correctly reporting on line 6 your year-end balance in traditional IRAs.
It's too late for the IRS to challenge your 2010 tax return, so there should be no concern about the IRS in the future assessing the tax that should have been paid in 2010.
Because your 2010 tax return treated your Roth conversion as including all $5,200 of your basis even though it was erroneous, because that was never corrected and cannot be corrected, you are not now permitted to take an inconsistent position that less than $5,200 of your basis was applied to the 2010 Roth conversion. Ultimately the result is the same as if you had no year-end balance in traditional IRAs at the end of 2010, you no longer have any basis.
