29, male public officer with 7 yrs of experience and NS

2007 - $1800 (started off as non-graduate officer in ministry)
2008 - $2000
2009 - $2200 (completed my part-time degree in IT at UniSIM)
2010 - $2900 (resigned and joined a SB at MX13)
2011 - $3100 (received B grade)
2012 - $3700 (received B+ grade & promoted to MX12)
2013 - $4000 (received B grade)
2014 - $4500 (received B grade)

Not sure of the max grade that I can hit if I work long enough in the public sector. I am currently doing part-time masters at NUS and is due to graduate in second quarter of next year. Thinking of joining the private sector next year for better opportunities. Anyone can advise?