Looks like you could move to a 1/7 split and the single SND will be able to keep up with the load. Everything looks like it is running pretty well based on the blades you have enabled, the zero templating rate (conns in fwaccel stats -s) is caused by Anti-bot and you can't really do anything about it.
One thing that is slightly high is your percentage of F2F traffic in the mid 20's. Ideally that should be 10% or lower, this F2F percentage could be caused by a large number of drops invoking excessive rulebase lookups which the penalty box should help with. Run the command fw ctl multik gconn and compare it with fwaccel conns (which does not show F2F connections), are there any common attributes of connections that are being handled F2F?
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