Performance: ipsec tranport mode over GRE or ipsec in tunnel mode?
Anyone know which method is better for performance/throughput?
Not likely to be a measurable difference.
Transport is the preferred method and it saves you 20 bytes but since the recommended MTU size is 1400 on both GRE + IPsec (Transport mode) = 1440 and GRE + IPsec (Tunnel mode) = 1420 there is no bigger difference between them.
Just don't forget to set MTU on GRE interface or you will lose data