The interest rate on secured IRS claims increased to 4%, effective with cases confirmed on or after April 1, 2016. The rate is adjusted by the IRS quarterly, but had remained at 3% for more than four years. The higher interest rate could make a difference in feasibility, so make sure you use the new interest rate in your calculations.
Per 11 U.S.C. § 511, the interest rate payable on secured tax claims is determined by applicable nonbankruptcy law as of the calendar month in which the plan is confirmed. You cannot modify the interest rate on a secured tax claim through the plan.
By the way, the IRS tax lien (if notice is properly filed) encumbers everything, including exempt property. 11 U.S.C. § 522(c)(2)(B). Keep that in mind when determining the secured value of the IRS’ secured claim.