You can remove the diff, but that won't necessarily change the time, depending on logs. You'll have more logs to restore.

Best way: copy the full to an external HDD and have someone take it to the other center. UPS/ FedEx, etc. Then once that's done, or in progress, take a diff and start that transferring, or log backups. You'll have to get all logs moved up to the point in time you want to recovery until.

There's no magic way to get more data through a slow pipe, or remove data from a restore sequence.