Hi there,
For the SRX3400:
Each SPC (single SPU for 3400) will do:
Packets per second: 1,100,000 PPS
New Connections Per Second: 50,000
Firewall throughput: 10Gbps
IPS throughput: 2.5Gbps
VPN throughput: 2.5Gbps
Now these numbers are old (Junos 9.4) but:
If you have 1 SPU with 1 NPC, you're limited to 10Gbps FW throughput at 1500 bytes or 2.5Gbps IMIX
If you have 2 SPUs with 1 NPC you're still limited to 10GBps FW throughput at 1500 bytes, but 5Gbps IMIX
If you have 3 SPUs with 2 NPCs, you can get up to 20Gbps FW throughput at 1500 bytes or 8Gbps IMIX
Depening on the mix of IDP/IPSEC these will vary too.
Hope this helps