F&F figures can be reduced to reflect administration savings. However, your current payment is pretty close to the bone anyway and the available 'wriggle room' reduced. Don't forget that a variation meeting to consider the offer will also be adding £300 - £500 to the costs.
That all said, I have seen low offers accepted before, as well as high ones refused. It is a difficuklt, if not impossible, call to make.
But, as Luluj says, there will also be the few payments between making the offer and it being accepted, which will knock 2 or 3 payments from the balance, bringing it much closer to that £2500 you are thinking of offering.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014