Your monthly payments are set by the level of disposable income you have, as calculated by your IP. Extra payments do not reduce the amount left to be paid, but just increases the amount that creditors get back. If you get your regular payments increased that will merely increase the amount required for early closure as creditors will then be expecting more.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk
If you have a third party offer of help, a full and final could offer early closure, however when making your offer ensure you stipulate the monies are only available to help conclude your iva early and the reasoning for it.
A full and final offer at year 2 or 3 should be based on payments left to pay x months remaining. You could reduce it a little to take into account early payment to creditors and less admin fees.
As foggy says, purely increasing your monthly payment won't end your iva any earlier.
Sharing from experiences of dealing with debt
There is a solution for everyone .... Just need to stay positive !