Salary from Highest to Lowest :-
New York > London > Hong Kong > Singapore

If you look at the websites of the Big 4 of SG, they are mostly staffed with lawyers from NUS. Whereas the "real" Big firms in NY, LN, HK is staffed with people from oxbridge and ivy league, so you are not really comparing apple with apple. I think it is difficult for an NUS lawyer to get employment in NY and LN, maybe HK is the easiest option if they are thinking of getting out of SG. But it is not easy, they have HKU Law over there and they are more familiar with the local law.

You will remember the case of the son of Senior Counsel Tan Chee Meng who admits evading NS. The Straits Times reported that "SC Tan remained in Singapore because of the lack of employment opportunities overseas".

s://.straitstimes.com/singapore/courts-crime/second-son-of-senior-counsel-tan-chee-meng-admits-evading-ns-for-about-6

Sorry for bursting your bubble NUS lawyers