What will be the argument for asking extra baggage allowance?
 

Out from the states I'll have no problem on the weight of my luggage since it'll be light, but on the way back, my itinerary now looks a bit messy.

CX 504 HKG=>NRT on Y

Stay in NRT for 1 day

SQ 12 NRT=> LAX on F

Stay in LAX for 1 day

UA 556 LAX=>SFO on Y

My luggage from Asia back to the U.S. may weight in between 23 to 25kg. I already foreseen that would be a problem while check in at CX counter & check in at UA counter...

Anyone had any experience on what will be the argument for asking extra baggage allowance?

Handing over a credit card usually works.

You mean pay the excess baggage fee even though just a couple kilos over...?

So flying SQ F & the status on Star Alliance Gold doesn't give me anything extra to that...

No - hand over the credit card and say it's okay to pay the extra charge. If you're only a bit over, they'll probably let it slide "just this once" just because they don't want to do to the paperwork. That's what happened to me at NRT once.

And if they do in fact take you up on your offer to pay the overweight baggage fee, keep in mind that if you are actually overweight, then you actually do owe the fee and don't be too bitter about it.