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A single MPLS label only adds 4 bytes, so you would lose that much, per packet to additional overhead. If you're packets are minimal size, that lost 4 bytes will lose you about 6%, but if you packets are large (1500B), you'll only lose .27%.
GRE/IPSec adds, depending on configuration, around 60 to 80 bytes, per packet. For minimal size packets, you're going to lose about half your bandwidth. For large (1500B) packets, you'll lose about 5.3%.
Depending on how you measure throughput, GRE/IPSec can be adverse to traffic, because, unlike MPLS, its overhead reduces the maximum possible payload, which can result in IP fragmentation issues, which further increases bandwidth lost to overhead.
