Yes, you get an extra 15kg on top of 30kg and an additional piece of baggage. Also, MH limits each passenger in all classes to 2 pieces of baggage excluding their Enrich status holders. oneworld status holders are also subject to this limitation, so as a sapphire, you would have a limit of 3 pieces.

The agents on the phone probably mistook your BA Silver with MH Silver.

If still in doubt or if you encounter problems at check in:
http://www.malaysiaairlines.com/my/e...allowance.html

Hope that helps.