Hi there and welcome to the forum. Overtime you are generally allowed to keep the first 10% and then it is split 50/50 to be paid in to your IVA. Therefore there is a benefit to you but has to be split. However, that said check your own proposal as some do differ in how they deal with this. Check with your proposed IVA company to ensure you fully understand your proposal before proceeding.
When in my IVA my husband's wage used to vary and I paid any uplift payments over monthly to avoid any surprises at review time.
IVA final payment left the bank on the 26th January 2013...looking forward to a debt free future.
Yes you usually have to pay 45% into the IVA but get to keep the rest. You can then potentially save this up and potentially use this to make a F&F settlement offer further down the line, depending on the figures involved.