I'm not very sure, but I would say your 8 years of experience would count for nothing much when you want to transfer over to the public sector. You may probably get a $200 - $400 increment, based on how hard you sell yourself, and whether your skill sets are transferable (e.g. HR, Procurement, Audit,and IT etc).

Private sector is better in the sense that if you work hard, you will be looked upon based on your results/accomplishments, and not your academic credentials. That is to say, if you work hard, there's a higher chance you'll be able to climb up the corporate ladder way faster when comparing to public sector.

With regards to your current salary, I would say that you've probably joined a wrong company to begin with, or the wrong industry. Though, I can't really give a fair judgement as you did not state your academic background, what you're working as, and in what company (SME or MNC).

My sincere advise is to jump to another private company that require your area of experience and expertise. Probably higher increment as opposed to public sector. Mind, there's a formula (from PSD) for deriving your expected salary in the public sector, and hence there are no such things as demanding more than 20% or even 10% increment.