The aggregated ethernet link speed does nothing to change the transmission speed of the interface. It is there to set a constraint so that if a member interface does not match (eg: a 1G interface only negotiates 100M) then it will not be included in the ae bundle.
What you are trying to do *should* work, however your metro ethernet provide may choose not to pass LACP PDUs, so you might need to make this just a static link aggregation.
The obvious downside to this is that if one circuit becomes unavailable, but your NTU still provides the EX link, you will start dropping traffic.