If you get a better job then obviously your payments to your IVA will be more but, unless you can pay off your total debts in full plus fees during that time, you're iva won't finish any sooner.
It may just mean that your creditors get a better dividend.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
Don't let the iva put you off taking a job with a higher salary just because you won't feel the full benefit of the increase in your salary. At the end of the iva, your career will have progressed in the direction you want and you will reap the rewards!!!
With a higher salary, all is not lost to creditors! You will recalculated your disposable income using the new salary, and then your IVA payments will increase by one-half of that new sum - thus allowing you to retain your 50% share. It may not be a lot but it all helps.
This assumes that your IVA has been presented in line with the IVA protocol. Best to check this out by having a read through your own documents or checking with your IP directly.