It really depends on the modifications and what has been written into the proposal. The statutory interest is a realisation and often creditors say that the IP must take 15% of realisations so it would be included in their fee.
Ultimately it should cost you nothing as if the IPs fee will come out of the statutory interest and reduce the amount payable to the creditors.
Many IP's have proposed variations to the creditors to get the statutory interest requirement removed as it does seem a trifle unfair to be further penalised for actually paying more back! Always worth asking.
Check your paperwork, though, in mine it was excluded from the start.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014